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1 Summary In the 20 years since publication of the Food and Nutrition Board's landmark report Matemal Nutr'izon and the Course of Pregnancy (NRC, 1970), the fields of nutrition and obstetrics have changed greatly. These changes, many of them stimulated by the report itself and the work of suc- cessor committees of the board and of the Institute of Medicine, include expanded private and governmental research efforts, enhanced teaching of perinatal nutrition to students in the health care professions, and a resurgence of interest in applied nutrition by clinicians in obstetrics and pediatrics. Also during the period, the Supplemental Food Program for Women, Infants, and Children (WIC) was established in the U.S. Depart- mentofAgriculture; the reports Preventing Low Birthweight (IOM, 1985) and Prenatal Care (IOM, 1988) were published by the Institute of Medicine; the National Commission for the Prevention of Infant Mortality was estab- lished; and the report Caring for Our Future: The Content of Prenatal Care was released by the Public Health Service (DHHS, 1989~. All these events have underscored the importance of nutrition during pregnancy and the perinatal period. Greater visibility of maternity services in the United States has also drawn attention to many gaps and weaknesses in knowledge about maternal nutrition and about how recent findings should be applied in prenatal care. To address these deficiencies, the Food and Nutrition Board (FNB) established the Committee on Nutritional Status During Pregnancy and Lactation late in 1987 to conduct a detailed assessment of the published data. The committee addressed its task by forming three subcommittees: 1
2 SUMAL4RY Nutritional Status and Weight Gain During Pregnancy, Dietary Intake and Nutrient Supplements During Pregnancy, and Nutritional Status During Lactation. The parent committee, in addition to coordinating the work of its subcommittees, has maintained close liaison with the FNB's Subcommittee on the Tenth Edition of the Recommended Dietary Allowances in order to maintain consistency in the work of the two groups. In general, the committee found few well-designed studies and little scientific evidence regarding many important issues. For this reason, areas needing further research are underscored in this publication. Careful attention to the information in these reports should help stimulate both research and practice toward a common goal of improving the health and well-being of mothers and children in the United States. This publication includes two reports. Part I is a critical evaluation of the data concerning nutritional status and weight gain during pregnancy. Part II contains an examination of the evidence on the need for nutrient supplements during pregnancy. A second publication covering nutrition during lactation will be released in late 1990. Each of these reports responds to specific requests from the Bureau of Maternal and Child Health and Resources Development of the U.S. Department of Health and Human Services, which funded the study. The committee anticipates that material presented in the three reports will serve as a basis for additional publications that provide a comprehensive, practical approach for delivering nutritional care curing pregnancy and lactation. PART I: NUTRITIONAL STATUS AND WEIGHT GAIN The overall goals of the Subcommittee on Nutritional Status and Weight Gain During Pregnancy were to analyze the scientific evidence per- taining to weight gain during pregnancy and to formulate recommendations for healthy gestational weight gain. The subcommittee was asked to address the following questions: . How do nutritional status prior to pregnancy and dietary intake during gestation influence the pattern and total amount of weight gain? · Which, if any, anthropometric measurements are useful in assessing nutritional status during pregnancy? · How should weight gain recommendations be modified for pregnant women of black, Hispanic, and Southeast Asian origin and for those under age 20 or over age 35? Part I deals only to a limited extent with nutritional care during pregnancy. It does not include many other elements of prenatal nutrition services, such as the evaluation and improvement of the nutritional quality of diet (briefly covered in Part II of this volume), and the importance of
SUMAL4RY access to a regular and adequate supply of nutritious foods, development of sound eating practices for the family, dietary adjustments for mothers with acute and chronic medical conditions, and breastfeeding promotion and education. Although the subcommittee limited its review to its charge, it recognized that such a broad spectrum of nutrition services is an important part of comprehensive maternal health care. The subcommittee began its work by tracing trends in selected aspects of prenatal care, maternal nutritional status, and the course and outcomes of pregnancy (e.g., fetal growth, birth weight, postpartum weight retention). It then undertook its major effort-an extensive critical review of the scientific literature. Epidemiologic and clinical evidence pertaining to determinants of weight gain and effects of weight gain on maternal and child health were examined. This included consideration of total gain, pattern of gain, and composition of the added tissue. The practicality and usefulness of anthropometric measurements in the clinical setting were assessed. The subcommittee paid close attention to new analyses from the 1980 National Natality Survey, which provides the only recent nationally representative U.S. data on weight gain during pregnancy, infant birth weight, and an assortment of maternal characteristics. Most of the literature reviewed pertained to women living in industri- alized nations. Data were more complete for whites than for nonwhites. The conclusions and recommendations presented in this report relate to healthy women in the United States; they have not been evaluated with respect to women in less developed countries or women who have recently emigrated from those countries to the United States. The subcommittee took particular care in clarifying its definitions and in examining the strengths and limitations of study methods. For example, the term prepreg~ancy nutritional status can have many meanings. The subcommittee agreed that prepregnanc~y weight for height is the simplest and most useful index for evaluating prepregnancy nutritional status in the clinical setting, while recognizing that this is an indirect measure of energy stores only. The measurements are relatively easy to make, and the approach provides a systematic method for distinguishing between women who weigh more because of their greater height and those whose greater weight reflects extra fat or lean body mass. Much more complex methods are required for defining other aspects of nutritional status. However, weight for height is applicable to most studies that address gestational weight gain. The meaning of gestational weight gain also received the subcommit- tee's attention. Comparisons among studies are complicated by the many different methods used to compute gestational weight gain. In this report, three types of gestational weight gain are discussed in detail, namely:
4 SUMMARY · total weight gain (weight just before delivery minus weight just before conception); net weight gain (total weight gain minus the infant's birth weight); and rate per week (weight gained over a specified period divided by the duration of that period in weeks). Attention is also given to different methods of comparing weight gain with standards in both clinical and research settings. The subcommittee considered gestational weight gain in relation to clinical care and to several maternal and infant outcomes, especially to birth weight. The emphasis on birth weight reflects its importance for child mortality, morbidity, and physical and mental performance. There is a relative lack of studies relating gestational weight gain to other important maternal and child health outcomes. A central question is whether or not gestational weight gain is causally associated with pregnancy outcomes such as fetal growth and postpartum retention of adipose tissue. Potential causal relationships were examined by applying standard epidemiologic terms and concepts to characterize the relationships between maternal factors, nutritional intervention, gestational weight gain, and maternal and child health. Factors that investigators have linked with gestational weight gain include maternal prepregnancy weight for height, prepregnancy weight, maternal height, ethnic background, age and parity, cigarette smoking, socioeconomic status, and energy intake. Analyses of data from the 1980 National Natality Survey made it possible to examine whether specified maternal characteristics are independently associated with weight gain. For example, the association between smoking and gestational weight gain can be tested while holding constant prepregnancy weight for height, ethnic origin, and other factors. However, even this large, relatively representative data set was not big enough for an examination of all the relationships of interest, such as the association of Hispanic origin with weight gain in obese women. For certain analyses of data from the 1980 National Natality Survey, the subcommittee used a gestational duration of 39 to 41 weeks and a live birth weight of 3 to 4 kg as an operational definition of favorable pregnancy outcome (recognizing that a small percentage of such infants may have serious birth defects or other health problems). The use of this range for birth weight represents a balance between the benefits of increased fetal growth for the infant, on the one hand, and the possible risks to the mother and infant of complicated labor and delivery with high birth weight (>4 kg), on the other.
SUMMARY s Historical Perspective Between the 196Os and 1980s, it gradually became common to recom- mend a gestational weight gain averaging 11 kg (24 lb) or more, rather than the 8 to 9 kg (18 to 20 lb) or less recommended previously. This change was accompanied by a 50% increase in gestational weight gain. Between 1971 and 1980, mean birth weight increased by approximately 60 g for whites and 30 g for blacks, low birth weight (<2.5 kg) prevalence was reduced by about 20% for whites and 7% for blacks, and the high birth weight prevalence increased by 30% for whites and by 15% for blacks. Several factors that may also have contributed to fetal growth or gestational weight gain during the period include increased prepregnancy weight, increased height, decreased smoking during pregnancy, increased participation in the WIC program, and earlier prenatal care. Conclusions on Weight Gain During Pregnancy Assessment of Weight and Weight Gain Prepregnancy weight-for-height and serial weight measurements are the only anthropometric measurements with documented clinical value for assessment of gestational weight gain. In the clinical setting, it is difficult for different individuals or even for the same person to obtain reproducible measurements of skinfold thicknesses. Even those skinfold thickness measurements that are reliable are not useful clinically, because there are no properly validated equations that use skinfold thicknesses to predict the total body composition of pregnant women, nor are there reference standards for skinfold thickness measurements validated against fetal outcomes. Body mass index (BMI), defined as weight/height2, is a better indicator of maternal nutritional status than is weight alone. The subcommittee used metric units (kilograms and meters) to calculate the BMIs used in this report. Weight for height expressed as a percentage of a standard is also usable. Since none of the weight-for-height classification schemes has been validated against pregnancy outcome, any cutoff points will be arbitrary for women of reproductive age. However, the subcommittee agreed on the following weight-for-height categories: · underweight: BMI <19.8 · normal weight: BMI 19.8 to 26.0 · overweight: BMI >26.0 to 29.0 · obese: BMI >29.0 The cutoff points generally correspond to 90, 120, and 135% of the 1959
6 SUMMARY Metropolitan Life Insurance Company's weight-for-height standards (Met- ropolitan Life Insurance Company, 1959)-the standards which have been in most common use in the United States. Although specific weight gain grids have substantial limitations, most are useful in the clinical setting because they permit visual tracking of weight gain by week of gestation. That is, they provide the practitioner with a visual impression of the progress of weight gain and simplify detection of an abnormal change in weight over time. Studies indicate that after the first trimester, the typical pattern of weight gain is one of gradual, steady increments, but definitive studies have not yet been conducted to determine what rates of gain are most desirable for favorable maternal and fetal outcomes and whether the optimal pattern of gain varies over the second and third trimesters. The subcommittee concluded that heavy emphasis should be placed on identifying major deviations in the rate of gain that may signal problems and warrant further assessment, as opposed to deviations related to errors in measurement or recording or to common shifts in weight related to fluid changes, contents of bladder and bowel, clothing, and time of day. Deviations from the expected pattern of weight gain may be entirely unrelated to nutrient intake and energy balance but, rather, may be related to such factors as those listed above. Gestational weight gain is normally attributable to increases in both lean and fat tissue of the mother and the fetus as well as to water retention. Most methods for assessing body composition (e.g., underwater weighing and total body water) are based on assumptions that have not been vali- dated for pregnant women and that may not be applicable because they do not distinguish between the added maternal and fetal tissues. As a re- sult, different methods-yield inconsistent results when applied in a research setting to the same population of pregnant women. Smooth, progressive weight gain generally represents a gain of lean and fat tissue, whereas errat- ically high weight gain is likely to represent excessive fluid retention. The clinical determination of ankle or leg edema (which worsens on standing) or generalized edema (which is not dependent on body position) can be useful in identifying extra fluid retention. However, it provides insufficient quantitative information about the amount of fluid that has been gained. Determinants of Gestational Weight Gain Prepregnancy weight for height is a determinant of gestational weight gain. On average, women who are overweight at conception (i.e., women whose BMI exceeds 26.0) gain less weight during pregnancy than do thinner women. However, there is wide variation in weight gain by women with normal pregnancies within each prepregnancy weight-for-height category. The variation is highest among obese women (BMI >29.0~.
SUMMARY 7 . . Some of the other maternal characteristics associated with an increased risk of low gestational weight gain (less than 7 kg, or 16 lb) occur in combination, e.g., low family income, black race, young age, unmarried status, and low educational level. These characteristics are also associated with shortened gestational duration. Studies using analytic methods to control for gestational duration and other factors suggest that black women are more likely to have low weight gain than are white women, but the reason for this difference is not known. Some ethnic groups with small average body size (e.g., Southeast Asians) have been reported to have low average weight gains, but the clinical significance of this has not been established. In studies of groups of women in the United States and elsewhere, energy intake is a determinant of gestational weight gain, but the reported relationship is weak. Changes in energy intake during pregnancy are difficult to detect because they are relatively small, on average, and current dietary assessment methods are rather imprecise. Variation in energy intake during pregnancy is determined largely by body size and the level of physical activity, not by gestational weight gain. Furthermore, energy intake may erroneously appear to be relatively unimportant for gestational weight gain if women expend less energy by decreasing their physical activity. Overall, however, there is no question that restriction of energy intake can limit weight gain or that excessive energy intake leads to extra fat storage. The impact of food supplementation on gestational weight gain (and fetal growth) appears to depend on the prior energy deficit of the woman and the extent to which the supplement makes up for the deficit between usual energy intake and requirements. That is, the impact is greater in women with low prepregnancy weight for height or in women whose food intake has been restricted. Consequences of Gestaizonal Weight Gain Wide variation is seen in weight gains among women giving birth to live, optimally grown (i.e., 3 to 4 kg at 39 to 41 weeks of gestation) infants. For example, in the United States in 1980, the 15th and 85th percentiles of weight gain were 7.3 and 18.2 kg (16 and 40 lb), respectively, for normal- weight women who delivered babies with these characteristics. This wide variation indicates that many factors in addition to weight gain during pregnancy contribute to a favorable outcome. Nevertheless, a large body of evidence indicates that gestational weight gain, particularly during the second and third trimesters, is an important determinant of fetal growth. Low gestational weight gain is associated with an increased risk of giving birth to a growth-retarded infant. This has important adverse consequences for subsequent somatic growth and, possibly, neurobehavioral development,
8 SUMMARY and it increases the risk of infant mortality. (More direct evidence also indicates a link between low weight gain and fetal and infant mortality.) The effect of first-trimester weight gain or weight loss on fetal growth is less clear, because the weight change is usually small, and because very few studies have included women in this trimester of pregnancy. The effect of gestational weight gain on fetal growth is modified by the mother's prepregnancy weight for height. Several epidemiologic studies, including the 1980 National Natality Survey, have convincingly demonstrated that the effect of a given weight gain (or rate of weight gain) is greatest in thin women and least in overweight and obese women. However, prepregnancy weight for height is a determinant of fetal growth above and beyond the effect of gestational weight gain: women who are thinner before pregnancy tend to have babies that are smaller than those of their heavier counterparts with the same gestational weight gain. Since higher birth weights generally present lower risks for the infants, desirable weight gains for thin women are higher than those for normal-weight women, whereas desirable weight gains for overweight and obese women are lower. Among obese women, the measured effect of weight gain on birth weight is weak. Most epidemiologic evidence suggests that maternal age does not modify the effect of weight gain on fetal growth. Very young adolescents (less than 2 years postmenarche) may, however, give birth to smaller infants for a given weight gain than do older women. Although the limited data indicate no clear modification of the effect by racial or ethnic background, black infants tend to be smaller than white infants for the same gestational weight gain of the mothers. Young girls and black women should therefore strive for weight gains toward the upper end of the ranges otherwise recommended for women with similar weights for height. Data concerning the effect of changes in maternal body composition on fetal growth are meager and inconclusive. Studies suggest that increases in maternal fat, lean tissue, and body water may each be associated with increased fetal growth. Very high gestational weight gain is associated with an increased rate of high birth weight which in turn is associated with some increase in the risk of fetopelvic disproportion, operative delivery (forceps or cesarean delivery), birth trauma, and asphyxia and mortality. These associations appear to be more pronounced in short women, i.e., <157 cm (62 ink. A lower ceiling on weight gain may therefore be preferable in short women at any given weight for height. Energy supplementation of pregnant women whose usual energy in- take is low relative to their needs may result in slightly higher average birth weights and decreased incidence of intrauterine growth retardation,
SUMMARY 9 although concurrent increases in gestational weight gain have not always been observed. Several, but not all, reports suggest that low rates of gestational weight gain are associated with a shorter mean gestational duration and an increased risk of preterm delivery. Difficulties in determining the length of gestation and assessing weight gain patterns prevent firm inferences, however. Gestational weight gain does not seem to be an important determi- nant of spontaneous abortion (miscarriage), congenital anomalies, maternal mortality, pregnancy-induced hypertension and preeclampsia, or volume or composition of milk produced during lactation. However, a sharp increase in weight accompanied by generalized edema and an elevated blood pres- sure remain the hallmarks of preeclampsia, a complication of pregnancy that requires immediate attention. On average, each successive birth adds about 1 kg (2.2 lb) of postpar- tum body weight above that normally gained with age. This gain is likely to be surpassed, however, in women with high gestational weight gains. In women carrying multiple fetuses, mean gestational weight gain appears to be greater by an amount larger than that accounted for by the weight of the additional fetuses and support tissues. In twin pregnancies, increased maternal weight gain also appears to be associated with increased birth weight. Clinical Recommendations The following recommendations are based largely on observational studies of weight gains in large groups of women and an attempt to balance the benefits of increased fetal growth with the risks of complicated labor and delivery and of postpartum maternal weight retention. In the absence of definitive data regarding optimal gestational weight gain, the subcommittee concluded that the target range for desirable maternal weight gain should be based on prepregnancy weight for height and should include the mean weight gain for women delivering full-term babies weighing between 3 and 4 kg. However, because the observed range for such mothers is too broad to be useful clinically, the subcommittee used its judgment in setting narrower target ranges by weight-for-height categories. Measurement Health care providers should adopt specific, reliable procedures for obtaining and recording weight and height and should implement them consistently in classifying women according to weight for height, setting weight gain goals, and monitoring weight gain over the course of pregnancy. Attention should be directed to the following elements:
10 SUMMARY TABLE 1-l Recommended Total Weight Gain Ranges for Pregnant Women?a by Prepregnancy Body Mass Index (BMI)b Recommended Total Gain Weight-for-Height Category kg lb Low (BMI < 19.8) 12.5-18 28~0 Normal (BMI of 19.8 to 26.0) 11.5-16 25-35 HighC (BMI > 26.0 to 29.0) 7-11.5 15-25 a Young adolescents and black women should strive for gains at the upper end of the recommended range. Short women (<157 cm, or 62 in.) should strive for gains at the lower end of the range. b BMI is calculated using metric units. c The recommended target weight gain for obese women (BMI > 29.0) is at least 6.8 kg (15 lb). · Prior to conception use consistent and reliable procedures to ac- curately measure and record in the medical record the woman's weight and height without shoes. These are the preferred bases for calculating prepregnancy weight for height. · Determine the woman's prepregnancy BMI. The table in Appendix C simplifies this calculation. The weight-for-height classifications shown in Able 1-1 are recommended. · Measure height and weight at the first prenatal visit carefully by procedures that have been rigorously standardized at the site of prenatal care. The initial weight measurement can be compared with prepregnancy weight and provides the baseline for monitoring weight change over the course of pregnancy. Measurement of height is recommended because objective data on height are often not recorded in the medical record. Use consistent, reliable procedures to measure weight at each subsequent visit. · Estimate the woman's gestational age from the onset of her last menstruation, preferably supplemented by estimates based on the obstetric clinical examination and, perhaps, by early ultrasound examination. · Record weight in a table and plot it on a chart included in the obstetric record, which should show the week of gestation on the hori- zontal axis and weight on the vertical axis. The subcommittee developed provisional charts (see Appendix B) showing the recommended target gain as the end points and the recommended rate of gain as the slope. Until a weight gain chart has been validated, the subcommittee favors use of these provisional charts. A notation should be made if gestational age is uncertain, since this can markedly affect placement of the woman's current
SUMMARY 11 weight on the chart. The charts are not meant to imply that the weight gain of all women of the designated BMI group should fall on the dashed line. Rather, the rate of gain should approximately parallel that shown by the dashed line. Counseling During pregnancy a woman may be particularly receptive to guidance regarding behaviors that may influence her health and that of her devel- oping fetus. The subcommittee recommends that women receive guidance regarding a healthy diet that will promote adequate weight gain. Sound dietary guidelines can be found in publications by federal agencies (e.g., DHHS/USDA/March of Dimes Birth Defects Foundation, 1982; USDA, 1979, 1989), by state agencies (e.g., Corruccini, 1977), and by private sources (e.g., American Red Cross, 1984; Dimperio, 1988~. · Set a weight gain goal together with the pregnant woman, prefer- ably beginning at the comprehensive initial prenatal examination, and explain to her why weight gain is important. This goal is best identified as a range of desirable total gestational weight gain and the rate of such gain. The subcommittee emphasizes use of a range rather than a single target weight, because a wide range of gestational weight gains is compatible with desirable pregnancy outcomes, because there is no method available for establishing the ideal gestational weight gain for an individual woman, and because a range rather than a single number may help alleviate exces- sive concern about weight gain during pregnancy. All women should be encouraged to gain enough weight to achieve at least the lower limit of weight specified for their weight-for-height category in Table 1-1. 1b help the woman achieve her weight gain goal, she should be given appropriate counseling or referred (e.g., to a social worker, dietitian, or WIC) to pro- mote consumption of a wholesome, balanced diet consistent with ethnic, cultural, and financial considerations. · Base the recommended range of total weight gain and pattern of gain mainly on prepregnancy weight for height. The subcommittee recommends that women of normal prepregnancy weight for height carrying a single fetus aim for a weight gain of between 11.5 and 16 kg (25 and 35 lb). This range is higher than that recommended in previous Food and Nutrition Board reports, which recommended a range of 9 to 11.5 kg (20 to 25 lb). The basis of this higher recommendation is the reduced risk of delivering an infant with intrauterine growth retardation with higher weight gains. However, the risk of maternal weight retention postpartum and fetal macrosomia may increase with higher weight gains in this range. A slightly higher target range of 13 to 18 kg (28 to 40 lb) is recom- mended for women with a low prepregnancy BMI (<19.8~. For those with
12 SUMMARY a high BMI (>26.0 to 29.0), the recommended range is lower 7 to 11.5 kg (15 to 25 lb). Short women should try to reach the lower end of the target weight gain range for their weight for height, and black and very young women should strive to gain weight at the upper end of the target range. Setting a realistic range for target weight at 40 weeks of gestation can be more of a problem if there is a question about the accuracy of prepregnancy weight. An appropriate approach for handling this situation is to determine gestational age and then focus on the rate of weight gain. The subcommittee set a 6.8-kg (15-lb) lower limit on gestational weight gain by extremely obese women (BMI >29.0), but it recognizes that many obese women with good pregnancy outcomes do gain less weight. Obese women should be encouraged to consume moderate amounts of nutritious food and a sufficient quantity of essential nutrients. All obese women should receive an individual dietary assessment and nutritional counseling. The recommended target total weight gain at term for women carrying twins is 16 to 20.5 kg (35 to 45 lb). There are insufficient data at this time to warrant different target gains based on prepregnancy BMI in a twin gestation. · For women with a normal prepregnancy BMI, recommend gain at the rate of approximately 0.4 kg (~1 lb) per week in the second and third trimesters of pregnancy. Underweight women (BMI <19.8) should strive to gain weight at a somewhat higher rate, i.e., 0.5 kg (or slightly more than 1 lb) per week and overweight women (BMI 26.0 to 29.0) should strive to gain weight at a somewhat lower rate of 0.3 kg (0.66 lb) per week. The target rate for extremely obese women should be determined on an individual basis. , ~ , Monitoring Progress Periodic prenatal care provides health care workers with opportunities for identifying potential problems and intervening early when indicated. The monitoring of weight gain is a key part of this process. . Monitor the prenatal course to identify any abnormal pattern of gain that may indicate a need to intervene. Assess the pattern of gain at each visit relative to the established weight gain goal and the course leading to that goal. A slightly lower or higher rate of weight gain than that recommended is not cause for alarm, as long as there is a progressive increase in weight that approximately equals the recommended rate of gain. Reasons for marked or persistent deviations from the expected pattern of gain should be investigated. In particular, gains of less than 0.5 kg (1 lb) per month for obese women and less than 1 kg (2 lb) per month for women of normal weight require further evaluation. Gains greater than 3 kg (6.5 lb) per month may also benefit from evaluation, especially
SUMMARY 13 after week 20 of gestation, but they should not be viewed as a reason to curb food intake sharply. Possible reasons for deviations from the normal pattern of gain include, in addition to inadequate or excessive food intake, measurement or recording errors, differences in clothing or time of day, accumulation of fluid, and multiple gestations. Health care providers are encouraged to assess the extent to which inaccuracies in measurement may have contributed to a seemingly abnormal rate of weight gain and to take corrective action. · When abnormal gain appears to be real, rather than a result of an error in measurement or recording, try to determine the cause and then develop and implement corrective actions jointly with the woman. For example, if a multiple gestation is identified, it would be advisable to revise the weight gain goal upward and support efforts to increase weight accumulation. If it appears that a lower than recommended weight gain is the result of an inadequate food supply or inappropriate self-restriction, corrective measures should be taken promptly. If evidence indicates that the abnormally high weight gain probably resulted from overeating, it would still be appropriate to revise the total weight gain goal upward to allow for the recommended rate of weight gain, but in this case it would be desirable to provide counseling to moderate food intake. Physical activity patterns should also be evaluated, and women should be encouraged to undertake or continue appropriate levels of activity. Research Recommendations The following research needs were identified during the literature re- view and subsequent discussions. These are not presented in order of priority. The subcommittee encourages investigators to give careful consid- eration to methodologic problems related to estimates of gestational weight gain, prepregnancy weight for height, and gestational duration (discussed in Chapter 4) when designing studies and interpreting results. Epidemiologic Research · Examine the effect of gestational weight gain (and the pattern of gain) on preterm delivery. · Examine the relationships between weight gain and maternal health. Investigate the relationship between high gestational weight gain and subsequent maternal obesity. · Identity the effects of overall gestational weight gain, weight gain patterns, and composition of gained tissue on pregnancy outcomes within specific ethnic groups and among very young women and women over age 35.
14 SUMMARY · Identify the characteristics of women who would benefit from in- creased energy intake. · Examine the effects of energy intake and weight gain on maternal and perinatal outcomes in markedly or moderately obese women. · Explore the use of different measures of gestational weight gain (e.g., net weight gain and rate of weight gain) in research on gestational weight gain and pregnancy outcomes. Basic Research · Determine the incremental dietary energy needs for pregnancy by measuring energy stores and expenditures over the course of gestation. · Identify appropriate animal models to use in investigations of the role of nutrition in human pregnancy, considering the number of fetuses, type of placentation, maturity of the fetus at delivery, body composition, and physiologic adaptations such as plasma volume expansion. · Investigate the influence of energy intake and prepregnancy weight- for-height status on patterns of gain of fat, lean, water, and total weight by using longitudinal studies beginning before conception and continuing throughout gestation. This requires research to develop new methods or modify existing ones for measuring body composition accurately during pregnancy · Identify the hormonal and biochemical determinants of weight gain pattern and composition. Applied Research · Develop and validate protocols and standards for use in the clinical setting. For example, establish cutoff values for prepregnancy BMIs as they relate to gestational weight gain and maternal and fetal outcomes. Develop a clinically useful weight gain chart and validate it against outcome data. · Improve methods of prenatal nutritional surveillance for public health purposes. For example, include prepregnancy weight and height on birth certificates and standardize assessment and reporting instruments used in government programs such as WIC. · Test recommended ranges of gestational weight gain against out comes. Test the effectiveness of specific interventions that are used to improve weight gain.
SUM~fARY 15 PART II: DIETARY INTAKE AND NUTRIENT SUPPLEMENTS In the United States, vitamin and mineral supplementation is common, especially among pregnant women. The Subcommittee on Dietary Intake and Nutrient Supplements During Pregnancy and the parent committee consider food as the optimal vehicle for delivering nutrients and nutrient supplementation as an intervention. As with other types of intervention, a recommendation to supplement the diet with special vitamin, mineral, or protein preparations should be based on evidence of a benefit as well as a lack of harmful effects. In addressing the advisability of supplementation, the subcommittee first reviewed biochemical, anthropometric, clinical, and dietary methods for measuring the adequacy of specific nutrients during pregnancy. It then identified nutrients that can be provided in adequate amounts by dietary means and those for which supplementation may be desirable. Special attention was directed toward protein, folate, iron, zinc, calcium, and vitamins that might exert toxic effects if taken in high doses. Evidence was sought concerning special recommendations for pregnant women of black, Hispanic, and Southeast Asian origin and for women in their teens or over age 35. Evidence regarding the potential value of periconceptional multivitamin supplements in the prevention of neural tube defects was evaluated. The interaction of diet with use of tobacco, alcohol, and caffeine was also reviewed. In view of the widespread use of marijuana, the epidemic of cocaine use among women of childbearing years, and cocaine's major adverse effects on health, the consideration of nonfood substances was enlarged to include those illegal drugs. Specific recommendations for nutrition counseling and other services to help improve maternal and family food intakes are beyond the scope of this report. Among the many sources of information on these topics are the National Center for Education in Maternal and Child Health in Washington, D.C., Cooperative Extension's Expanded Food and Nutrition Education Program (operated at the county level), state WIC programs, state departments of health, the American Red Cross, and the American College of Obstetricians and Gynecologists. Methodology 1b determine whether nutrient supplements should be recommended during pregnancy, the subcommittee examined several lines of evidence. These included the results of controlled experimental studies of nutrient requirements of women, laboratory studies and functional tests of nutri- ent status, epidemiologic studies linking diet or supplement use with various
16 SUMMARY pregnancy outcomes, dietary intake data, and in some cases, studies in which animal models were used to examine the effects of nutrient deficiency or excess. In its initial deliberations, the subcommittee determined that it would limit its review to trace elements and vitamins for which a human re- quirement has been established and to calcium, magnesium, and protein. Protein is not an ingredient of vitamin-mineral preparations but is avail- able in special powders and formulas as a dietary supplement. Essential micronutrients excluded from consideration were phosphorus, sodium, chlo- ride, and potassium all of which are widely available in foods but are not ordinarily included in multivitamin-mineral preparations. Limitations in the Data Maternal physiologic changes during pregnancy affect the results of laboratory tests and of tests reflecting how well the body is functioning (e.g., enzyme activity and cellular uptake of nutrients). Reference standards and cutoff points for laboratory and functional tests are affected by normal pregnancy to degrees that vary with the stage of gestation. Review of dietary intake data provides one method of determining which nutrients warrant close attention. Less likely candidates for routine supplementation are nutrients consumed at levels close to the 1989 Recom- mended Dietary Allowance (RDA) for pregnant women, especially if there is no evidence that a sizable segment of the pregnant population has intake falling substantially below the RDN On the other hand, average nutrient intakes lower than the RDA were viewed as inadequate evidence to support routine supplementation of pregnant women with that nutrient. Because the RDAs for most minerals and vitamins include a wide margin of safety, the needs of many pregnant women can be met with intakes below the RDN Moreover, estimates of nutrient intake based on dietary intake data are imprecise and tend to underestimate total food and nutrient intake. The data indicate that, on average, dietary intake by pregnant women is less than the RDA for eight nutrients: vitamins Be, D, E, and folate; iron; zinc; calcium; and magnesium. Since nutrient supplements typically contain multiple nutrients, there is the potential for nutrient-nutrient interactions during absorption and metabolism. An increase in the concentration of one nutrient may adversely affect the availability, absorption, or utilization of other nutrients provided by the supplement and by diet. The use of such substances as tobacco, alcohol, caffeine or coffee, marijuana, and cocaine may affect maternal nutrition in two general ways.
SUMMARY 17 The substance may increase the actual need for one or more nutrients by a variety of mechanisms, for example, by increasing urinary excretion, or it may lead to undesirable changes in food and nutrient intake. C?~terza Used in Formulating Recommendations The subcommittee decided to recommend routine supplementation only if the usual dietary intake of the nutrient is likely to be low enough to limit the production of compounds essential for body function or to adversely affect the health of the mother, fetus, or newborn and if sup- plementation poses no known dangers for the mother or fetus. Data on nutrient interactions were considered in formulating recommendations for specific nutrients and combinations of nutrients. In the subcommit- tee's view, dietary supplements should not replace dietary counseling or a well-balanced diet. Improvement of diet quality through use of nutritious foods is strongly preferred to supplementation. Foods supply energy and essential nutrients not found in supplements, and there is less risk of unde- sirable nutrient-nutrient interactions when nutrients are provided by foods. Nonetheless, certain situations may warrant use of a multivitamin-mineral supplement, as described later in this chapter. Conclusions The subcommittee concluded that evaluation of a pregnant woman's dietary pattern by a food history or food frequency questionnaire, aug- mented by questions about special problems or conditions that might affect her dietary adequacy or needs, may provide the best information on which to assess the need for nutrient supplementation. Except for tests for hemoglobin (or hematocrit) and, possibly, serum ferritin, it is impractical to use other laboratory and functional tests to assess nutrient status in routine prenatal care. After an in-depth review of dietary intake data for women in the United States and evidence from clinical, metabolic, and epidemiologic studies, the subcommittee concluded that iron is the only known nutrient for which requirements cannot be met reasonably by diet alone. 1b meet the increased need for iron during the second and third trimesters of pregnancy, the average woman needs to absorb approximately 3 mg of iron per day in addition to the amount of iron usually absorbed from food. Evidence from iron absorption studies indicates that low-dose supplements (e.g., 30 mg of ferrous iron daily) can provide this amount of extra iron. To lines of evidence contributed to the conclusion that supplementa
18 SUMAL4RY lion with iron is advisable: need for iron is high in relation to usual dietary supply, and iron-supplemented pregnant women have higher hemoglobin levels than do unsupplemented women. Iron meets the subcommittee's criterion that intake is likely to be low enough to limit the production of a compound hemoglobin-that is essential for body function. Evidence that iron deficiency adversely affects maternal and fetal health is only suggestive. Because iron at low doses poses no known dangers to the mother or fetus, the subcommittee concluded that in populations that commonly have iron deficiency, the potential benefits of iron supplementation outweigh the risks. Low-dose iron supplements offer distinct advantages over higher-dose ones: less potential for undesirable nutrient-nutrient interactions, more efficient absorption, and less risk of causing gastrointestinal distress. Pregnant women can meet the physiologic requirements for folate from diet by following dietary guidelines such as those provided in the publications listed at the end of this chapter (American Red Cross, 1984; Corruccini, 1977; DHHS/USDA~arch of Dimes Birth Defects Foundation, 1982; Dimperio, 1988; USDA, 1979, 1989~. Folate deficiency now appears to be very rare among pregnant women in the United States. The extent to which the common practice of folate supplementation has contributed to the rarity of this deficiency is unknown. In the past, particularly during the l950s and 1960s, folate deficiency was identified in some women in industrialized nations, including the United States and the United Kingdom. Pending further research, the subcommittee considers it prudent to supplement the diet with low amounts of folate if there is any question of adequacy of intake of this nutrient. Evidence is not sufficient to conclude that routine supplementation with other nutrients is warranted, although clearly there are situations re- quiring special consideration. For example, certain dietary practices that restrict or prohibit the consumption of an important source of nutrients, such as avoidance of all animal foods or of vitamin D-fortified milk, increase the risk of inadequate nutrient intake. In such cases, if diet quality cannot be improved through better food selection, selective supplementation may be desirable. There is evidence that women carrying more than one fetus and those who use cigarettes, alcohol, or illicit drugs have increased re- quirements for certain nutrients. Furthermore, adolescents and drug users often follow dietary practices that lead to low intakes of a number of nutrients. Because protein is abundant in usual diets in the United States and because of evidence suggesting possible harm from routine ingestion of specially formulated high-protein supplements, the use of special pro- tein powders or formulated high-protein beverages should be discouraged.
SUMMARY 19 However, there is no contraindication to increased use of food sources of protein such as milk and flesh foods as part of a well-balanced diet, especially because these foods are also rich sources of vitamins and minerals. Although some studies in nonpregnant populations suggest that caf- feine intake may decrease the availability of certain nutrients such as calcium, zinc, and iron, there is only inconsistent and fragmentary evidence that the consumption of coffee or caffeine during pregnancy exerts adverse effects on the fetus. It nevertheless appears sensible to limit the consump- tion of caffeine-containing products during pregnancy, although data are insufficient for setting a specific limit on intake or for recommending nutri- ent supplementation for women who continue to consume caffeine during pregnancy. Special recommendations on the basis of ethnic background alone do not seem to be warranted at present. There is no evidence of substantial differences in nutrient requirements among various ethnic groups, but ethnic differences in food choices and, consequently, in the mean intake of certain nutrients do exist. For example, because ethnic groups with a high prevalence of lactose intolerance customarily consume relatively low amounts of milk, their intakes of calcium and vitamin D deserve special attention. The adequacy of calcium and vitamin D intake among pregnant women under age 25 also deserves special attention, since bone mineral density is still increasing during that period of life. For older women who become pregnant, there is no evidence that high calcium intake will Drotect against later bone loss caused by osteoporosis. There is some evidence that periconceptional use of multivitamins or folate may provide some protection against the occurrence of neural tube defects. Data to support a recommendation to use periconceptional vitamins for this purpose are not conclusive at this time. Because of accumulating data that excessive vitamin A consumption poses a teratogenic risk, supplementation with preformed vitamin A should be avoided during the first trimester unless there is specific evidence of a deficiency. Carotene intake need not be restricted. Clinical Recommendations Dietary Assessment Routine assessment of dietary practices is recommended for all preg- nant women in the United States to allow evaluation of the need for improved diet or vitamin or mineral supplements.
20 Iron SUMMARY For the general population of pregnant women, supplements of 30 mg of ferrous iron are recommended daily during the second and third trimesters. This amount of ferrous iron is provided, for example, by approximately 150 mg of ferrous sulfate, 300 mg of ferrous gluconate, or 100 mg of ferrous fumarate. Administration between meals or at bedtime on an empty stomach will facilitate iron absorption, but taking ascorbic acid with supplements containing ferrous iron does not enhance iron absorption. Folate Although routine folate supplementation of pregnant women is not recommended, a supplement of 300 ,ug/day may be given when there are doubts about the adequacy of dietary folate. Women who ingest fruit, juices, whole-grain or fortified cereals, and green vegetables infrequently are likely to have low folate intake. Multivitamin Mineral Supplements For pregnant women who do not ordinarily consume an adequate diet and for those in high-risk categories, such as women carrying more than one fetus, heavy cigarette smokers, and alcohol and drug abusers, the subcommittee recommends a daily multivitamin-mineral preparation containing the following nutrients beginning in the second trimester: Iron 30 mg Vitamin Be 2 mg Zinc 15 mg Folate 300 fig Copper 2 mg Vitamin C 50 mg Calcium 250 mg Vitamin D 5 ,ug To promote absorption of these nutrients, the supplement should be taken between meals or at bedtime. Nutrient Supplementation in Special Circumstances As mentioned above, supplementation of other nutrients may be de- sirable for certain pregnant women in the United States. The following are the subcommittee's recommendations for those special circumstances. Vitamin D: 10 ,ug (400 IU) dad) for complete vegetarians (those who consume no animal products at all) and others with a low intake of vitamin D-fortified milk. Vitamin D status is a special concern for women at northern latitudes in winter and for others with minimal exposure to sunlight and thus reduced synthesis of vitamin D in the skin. Calcium: 600 mg daily for women under age 25 whose daily dietary calcium intake is less than 600 ma. 1b enhance absorption and limit
SUMMARY 21 interaction with iron supplements, the calcium supplement should be taken at mealtime. There is no evidence that older pregnant women (i.e., those over age 35) have a special need for supplemental calcium. Vitamin B`2: 2.0 fig daily for complete vegetarians. Zinc and copper: When therapeutic levels of iron (>30 mg/day) are given to treat anemia, supplementation with approximately 15 mg of zinc and 2 mg of copper is recommended because the iron may interfere with the absorption and utilization of those trace elements. Research Recommendations Surer Needs · Representative data should be collected on the nutrient status of pregnant women and their usual dietary and supplement intake, especially with regard to iron and folate. Pregnant women should be oversampled in national surveys (e.g., the National Health and Nutrition Examination Survey and the Continuing Survey of Food Intake of Individuals) to improve the data base regarding health and usual nutrient intake in relation to age, income, and ethnic background. Nutritional Assessment · Special purpose longitudinal studies should be conducted from be- fore pregnancy to parturition to relate food and nutrient intake of individ- ual women to maternal and fetal nutritional status and pregnancy outcome. Priority should be given to iron; zinc; copper; calcium; magnesium; and vitamins D, B6, and folate. · Better practical diagnostic tests should be developed for detecting deficiencies of the following in pregnancy: folate, zinc, copper, vitamin B6, calcium, and magnesium. · Further work is needed to determine the intakes of nutrients by pregnant teenagers in specific age and economic groups and by women of different ethnic backgrounds. · In view of the increasing number of women who bear more than one fetus, it is important to acquire data on their food and nutrient intakes in order to develop appropriate nutritional interventions. · Strategies are needed for investigating the nutritional consequences of nausea, vomiting, and food cravings and aversions to provide data that are useful for determining the appropriateness of nutritional interventions for these conditions. · Itials should be conducted to assess the effects of nutrient-nutrient interactions on the absorption and utilization of specific nutrients when they are included in multinutrient supplements.
22 SUMMARY Iron Supplementation · Studies are needed on the prevention of iron deficiency without the use of iron supplements. Such methods would include counseling in the selection of diets to enhance iron absorption. The effectiveness of iron-fortified foods in preventing iron deficiency requires further study, and the improvement of fortification methods should be given a high pnorib. · Randomized, controlled trials are recommended to study the effects of iron in 15- to 60-mg doses in different formulations that include iron compounds alone, with folate, and in multinutrient preparations. Periconcepiu~nal Supplements Further studies should be conducted to investigate the effectiveness of routine periconceptional use of vitamins and minerals in the prevention of birth defects. Supplementation for High-Risk Groups Possible benefits of vitamin-mineral supplementation should be stud- ied for substance abusers, adolescents, and other groups at high risk of nutritional deficiency. REFERENCES American Red Cross. 1984. Better Eating for Better Health: Participant's Guide. I'm Pregnant: What Should I Eat? American Red Cross, Washington, D.C. 10 pp. Corruccini, C G. 1977. Nutrition for Pregnancy and Breast Feeding: Eating Right for Your Baby. Maternal and Child Health Branch, Family Health Services Section, California Department of Health Services, Sacramento, Calif. 20 pp. DHHS (Department of Health and Human Services). 1989. Caring for Our Future: the Content of Prenatal Care. A Report of the Public Health Service Expert Panel on the Content of Prenatal Care. Public Health Service, U.S. Department of Health and Human Services, Washington, D.C. 125 pp. DHHS/USDA/March of Dimes Birth Defects Foundation (Department of Health and Human Sen~ices/U.S. Department of Agriculture/March of Dimes Birth Defects Foundation). 1982. Food For the Teenager: During and After Pregnancy. DHHS Publ. No. (HRSA) 82-5106. Public Health Service, U.S. Department of Health and Human SeIvices, Rockville, Md. 31 pp. Dimperio, D. 1988. Prenatal Nutrition: Clinical Guidelines for Nurses. March of Dimes Birth Defects Foundation, White Plains, N.Y. 134 pp. IOM (Institute of Medicine). 1985. Preventing Low Birthweight. Report of the Committee to Study the Prevention of Low Birthweight, Division of Health Promotion and Disease Prevention. National Academy Press, Washington, D.C. 284 pp. IOM (Institute of Medicine). 1988. Prenatal Care: Reaching Mothers, Reaching Infants. Report of the Committee to Study Outreach for Prenatal Care, Division of Health Promotion and Disease Prevention. National Academy Press, Washington, D.C. 254 PP. Metropolitan Life Insurance Company. 1959. New weight standards for men and women. Stat. Bull. Metrop. Life Insur. Co. 40:14.
SUMMARY 23 NRC (National Research Council). 1970. Maternal Nutrition and the Course of Pregnancy. Report of the Committee on Maternal Nutrition, Food and Nutrition Board. National Academy of Sciences, Washington, D.C. 241 pp. USDA (U.S. Department of Agnculture). 1979. Food. Home and Garden Bulletin No. 228. Science and Education Administration, Food and Nutrition Service, U.S. Department of Agriculture, Alexandria, Va. 65 pp. USDA (U.S. Department of Agriculture). 1989. Preparing Foods & Planning Menus Using the Dietary Guidelines. Home and Garden Bulletin No. 232-8. Human Nutrition Information Service, U.S. Department of Agriculture. U.S. Government Printing Office, Washington, D.C. 31 pp.
