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4 Assessment of Gestational Weight Gain In this chapter, methods and issues related to the assessment of mater- nal weight gain during pregnancy are discussed. The types of data essential to assessment of gestational weight gain in the clinical setting are identified, and desirable methods of collecting, using, and interpreting these data are described. The subcommittee then discusses various definitions of gesta- tional weight gain and other factors that have complicated interpretation of the research on health implications of gestational weight gain. METHODOLOGICAL ISSUES RELATED TO DATA COLLECTION IN THE CLINIC When weight and weight gain are used to screen individual women for special treatment or to monitor the progress of a pregnancy, the consequences of misdiagnosis resulting from poor data quality may be substantial. Therefore, major emphasis should be placed on establishing and implementing data collection techniques that provide the most reliable and accurate data possible for the particular clinical setting. Clinical data relevant to gestational weight gain include weight, height, and gestational age. Other measures, such as skinfold thickness and indica- tors of frame size, are also occasionally recommended but are not included in this critique. For reasons discussed in Chapter 6, the measurement of skinfold thickness to assess composition of weight gain and nutritional status during pregnancy has not been thoroughly studied or validated. Similarly, there have been no studies on pregnant women to validate the 63

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64 NUTRITIONAL STATUS AND WEIGHT GAIN functional meaning of frame size indicators (e.g., elbow breadth and wrist circumference or diameter measurements suggested as a basis for improv- ing weight-for-height classification schemes). Therefore, the subcommittee believes it is premature to recommend specific techniques for making these measurements. Before making any of the measurements described below, one should consult a reliable source such as Lohman et al. (19~) for a thorough review of methods and techniques. Weight Weights measured in the clinic are preferable to self-reported or re- called weights. However, measurement alone does not ensure accuracy. Factors that can affect measurements of a woman's weight include the type of scale used (e.g., spring or beam balance scale), the accuracy of its cali- bration, the clothing worn by the woman, the time of day the measurement is made, and the contents of the bowel and bladder at that time. When considering efforts to improve the reliability of measurements, it is useful to keep in mind their purpose. Control of errors is most crucial when weights are collected over a short period (e.g., 1 to 2 weeks). For example, normal weight gain during the second trimester is approximately 0.4 to 0.5 kg (~1 lb) per week, or 1.6 kg (3.5 lb) per month, but it is not unusual for the accumulated errors in weekly or monthly weighings to total 0.4 to 0.7 kg (1 to 1.5 lb) in many clinics. Since a subnormal gain is often defined as less than 1 kg (2 lb) per month, with this degree of error, it might be very difficult to distinguish abnormal weight gains from measurement error over relatively short intervals. Subjects should be weighed on a platform beam balance scale with movable weights or on a high-quality electronic scale. The scale should be graduated to the nearest 100 g (0.25 lb) and calibrated periodically against a known weight or series of weights approximating the range of weights encountered in clinic patients. For example, barbell weights or boxes of books with known (recorded) weights can be stacked to cover the range of calibration weights. Subjects need not be weighed while they are nude, but procedures should be followed to promote consistency in the amount of clothing worn by all patients. The fewer clothes the better, as long as the weight of the clothing can be kept relatively constant for all women and all seasons of the year. This requires weighing women without purses, shoes, boots, coats, jackets, or any accompanying young children. All personnel responsible for taking weights and other measurements should be trained in the standard procedures chosen by the clinic. It should not be assumed that everyone knows how to obtain weight measurements properly. For clinical management of the patient, it is most important to have accurate measurements taken sequentially throughout the pregnancy.

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ASSESSMENT OF GESTATIONAL WEIGHT GAIN 65 Prepregnancy weights are determined primarily to establish whether the patient's weight is high, low, or normal and to provide a basis for identifying extreme first-trimester weight changes. Prepregnancy weight is best determined from objective data from the patient's chart reflecting a recent preconceptional visit. Self-reported Prepregnancy weights must be evaluated for plausibility and discarded if they are suspect. Height Knowledge of standing height (stature) is indispensable when careful classification by Prepregnancy weigh-l status is desired. Such classifications are based on body mass index (BMI; see the section Assessing Prepregnancy Weight for Height below) or on a percentage of a standard reference weight for a given height, e.g., the Metropolitan Life Insurance (MLI) Company standards for ideal body weight (1959, 1983~. Height can be measured with reasonable precision in a clinical setting if standard procedures are followed. A low-cost stadiometer can be constructed from a metal measuring tape and a headboard. The tape is attached to a vertical wall against which the patient can stand erect with the back, buttocks, and heels against the tape. The headboard can be constructed from two pieces of wood joined to form a right angle. The vertical board is pushed flat against the wall and lowered along the tape measure until the horizontal board makes contact with the patient's head. The measurement is taken at the point where the horizontal board touches the patient's scalp. Commercially available stadiometers can also be used, but the ones built into weighing scales are unsuitable for research purposes. The stadiometer should be inspected periodically to ensure that its position has not changed with use. Height should be measured as early in the pregnancy as possible to avoid errors caused by body posture changes, which may affect measurements beginning approximately 20 weeks into gestation. Postural change may reduce measured height by approximately 1 cm (0.4 in.~. Gestational Duration The adequacy of accumulated weight gain or achieved weight status at any given stage of pregnancy can be determined only when the length of gestation is known. An accurate estimate of length of gestation is also essential for research on gestational weight gain and fetal growth. By far the most common method of estimating length of gestation is calculation of the time since the last menstrual period (LMP) based on dates provided by the woman at her first prenatal visit. There is an established literature (e.g., Oates and Forrest, 1984; Wilcox and Homey, 1984) dealing with the accuracy of recall of this date. Recall becomes increasingly problematic as

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66 NUTRITIONAL STATUS AND WEIGHT GAIN the LMP becomes more distant in time and memory becomes less reliable. Also, a small percentage of conceptions in the United States occur during amenorrheic cycles. In some pregnancies, vaginal bleeding occurs within the first 4 to 6 weeks after conception and is falsely reported as a menstrual flow. Moreover, the date of the LMP does not denote the beginning of the pregnancy but only the presumed beginning of the cycle that produced the ovum that was fertilized 10 to 14 days later. Actual gestational length, or time from conception, is rarely known and is even less likely to be accurately self-reported several months after conception. Actual length of gestation is generally calculated from the estimated date of conception up to the date of the prenatal exam or delivery, whichever is applicable. In the clinic, a gestational age calculator or table, special computer program, or programmed hand calculator should be used to estimate the length of time since last menstruation, rather than calculations made in the head or on paper. Early (prior to 20 weeks) ultrasound measurement of the fetal biparietal diameter represents an alternative for estimating gestational duration. Although errors in estimating gestational duration may pose problems of interpretation when one uses incremental weight gain from conception to a point later in gestation, they do not do so when estimating the rate of weight gain between two or more prenatal visits after the first trimester. A shift of several weeks in the gestational age estimate does not affect the interpretation of the rate of gain, since it is generally linear over a broad range of pregnancy from 13 to 35 whelp of gestation. Assessing Prepregnancy Weight for Height ~ assess prepregnancy weight for height, it is necessary either to calculate BMI or to compare the weight for height to a weight-for-height reference standard (e.g., MLI ideal weight tables). ~ minimize the chance of calculation errors, it is advisable to use the units of measure used in the clinic (i.e., metric or English system). BMI is the most common expression of body weight corrected for height in use today. It is computed as follows: BMI = ht2 = g2 x 1~, or i 2 x 1~. BMI can be easily determined from the table in Appendix C. A similar weight-for-height index is achieved by expressing body weight as a percentage of desirable body weight adjusted for height in the following form:

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ASSESSMENT OF GESTATIONAL WEIGHT GAIN Percentage of reference weight = observed weight x 100 expected or desirable weight given the patient's height 67 The most widely used values for bracketing normal weight seem to be 90 and 120% of the 1959 MLI (Metropolitan Life Insurance Company, 1959) reference weights. This, in turn, corresponds to a metric BMI of approximately 19.8 to 26.0. However, some clinicians use 80% of the 1959 MLI reference as the cutoff point for distinguishing those who are underweight, while others may use 95% as the cutoff point. The cutoff point for overweight may be as low as 110% to as high as 130% of MLI reference weights. Moreover, an increasing number of clinics are using the 1983 version of the MLI reference weights (Metropolitan Life Insurance Company, 1983~. There is no statistical or scientific basis for prescribing one set of cutoff values or reference standards over another when assessing prepregnancy weight. The relationship between prepregnancy weight and various fetal or maternal outcomes is generally considered to be linear, with no well- defined threshold at either end of the prepregnancy weight distribution, but this view has not been verified by research. Pending further research, the subcommittee recommends using a metric BMI of 19.8 to 26.0 as the definition of normal (moderate) prepregnancy weight for height. This BMI range (see Appendix C) encompasses the 25th to 75th percentiles of prepregnancy weight for height of women in the 1980 National Natality Survey (Kleinman, 1990~. BMI is a preferred indicator of nutritional status because it depends on two commonly measured aspects of body morphology-weight and height. BMI is often viewed as a proxy for more accurate indicators of body composition such as body fat content or lean body mass, but it is not simply an obesity index. Women with BMIs greater than 26.0 do not necessarily have excess body fat deposition; in some, the extra weight may be attributable predominantly to muscle tissue or skeletal mass. Studies relating BMI or its equivalent to estimates of body fat obtained from more complex methods, such as those described in Chapter 6, have indicated a good correlation between percentage of body fat and BMI (e.g., R = .7; Jackson et al., 1988), but there will be considerable misclassification of undernutrition or obesity. Using Weight Gain Data in the Clinic It is desirable to plot cumulated weight gain (weight gain to date) sequentially on a weight gain chart that has a reference curve that reflects

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68 NUTRITIONAL STATUS AND WEIGHT GAIN normal weight gain. This allows visual inspection of the overall trajectory or slope of the patient's weight gain and comparison with the standard. It thus provides the clinician with a visual impression of whether or not weight gain is progressing normally for the mother. Award the end of gestation, cumulated weight gain becomes increasingly predictive of birth weight because the weight contributed by the fetus becomes relatively greater. Gestational weight gain can be monitored in a manner similar to methods recommended for monitoring child growth (see Griffiths, 1985~. For example, at the third prenatal visit it is useful to compare the rate of gain between the second and third visits to the rate of gain between the first and second visits. A rapid weight gain is more acceptable if the starting weight was unusually low for the woman's height or if the weight gain between the first and second visits was unusually low. In both cases, one might interpret the rapid weight gain as catching up to or compensating for poor earlier weight status. In contrast, if a woman were obese before pregnancy and she gained weight rapidly between each successive measurement, there is cause for concern. Unusually high or low gains should be checked for possible measurement or recording error. To determine numerically whether the rate of weight gain is close to that recommended, the following equation can be used: wet-wit_ weight gain = where wt is weight, GA is gestational age in weeks, ~ is time of most recent measurement, and t-1 = time of previous measurement. As an example, if body weight is 135 lb at week 20 of the pregnancy and 139 lb at week 25, then weight gain (lbhvk) = 25 k 20 k = 5 k = 0 8 lb~wk. It is less useful to calculate rate of gain for periods including part or all of the first trimester, when the rate of gain is normally low, than for periods within the second and third trimesters. Weight Gain Charts Many different weight gain charts and tables are available for use in the clinical assessment of gestational weight gain; Table 4-1 presents se- lected examples representing a wide range of applications. An evaluation of the characteristics of these various charts is useful in considering their application in the clinical setting. Weight gain charts evolved over the past

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ASSESSMENT OF GESTATIONAL WEIGHT GAIN 69 40 years in four basic stages: (1) early, normative charts constructed from a homogeneous sample (usually a very small sample) of healthy women who delivered normal infants, (2) modification of the early normative charts to account for maternal prepregnancy weight, (3) elaboration of the charts that incorporate prepregnancy weight status by inclusion of more recent normative data on gestational weight gain, and (4) elaboration of prepregnancy weight classification to establish target weights based on some percentage of the ideal or reference prepregnancy weight. However, there is still a need for a validated chart, with the characteristics outlined in Appendix ~ In Table 4-1, the large number of charts from the 1980s reflects an increased interest by the scientific community and a growing awareness of the inadequacy of early weight gain charts among practitioners. Also, the expansion of prenatal services in the Supplemental Food Program for Women, Infants, and Children (WIC) created a demand for gestational weight gain charts. The subcommittee had access to weight gain charts used by WIC in 21 states. They represent a broad range of different types of charts, based on different sources of information on normal weight gain. As described in Table 4-1, most of the charts developed for WIC at the state level include the normative curve originally reported by Tompkins and Wiehl (1951) (Figure 4-1) and make some allowance for prepregnancy weight for height. The mean total gestational weight gain for the 60 women in Tompkins and Wiehl's sample was 10.9 kg (24 lb), but no ranges, standard deviations or nercentile.s were renort~`l ~ O The curio developed by Tompkins and Wiehl has been used differ- ently in different charts. Some states (e.g., Ohio, Georgia, Oregon, Mas- sachusetts, New Hampshire, and Rhode Island) use the curve to represent the average or target weight gain for women with normal prepregnancy weight. In the chart used by Oregon WIC (Oregon WIC Staff, 1981) (Fig- ure 4-2), upper and lower limits have been added to the original Tompkins and Wiehl cube, but their derivation is not given. Other states use different curves for women who are overweight or underweight before pregnancy (e.g., Figure 4-3~. Some states (e.g., Wisconsin, Florida, New Jersey, Idaho, Washington, Wyoming, and New York) use the Tompkins and Wiehl curve to depict the lower limits of normal. Some use separate charts for normal weight, overweight, and underweight women, and also include upper and lower limits, such as those in Figure 4-4A through D. The limits have not been based on systematic studies of the consequence of misclassification resulting from the use of specific cutoff points or limits such studies have not yet been conducted. Dimperio (1988) has developed a weight gain chart which includes an 11.4-kg (25-lb) total gain marking the lower limit and a procedure for adjusting the single curve for different prepregnancy weights for height

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70 v, o cq - 'e ct a) At ct ~- ct - au 3 - o ._ A_ Cal o U) Cal ._ Cal ._ so au Cal Ct Ct .= 3 Ct ._ ~ , to .= Em ~ o ._ o ~ Cal .o o - ~4 Cal - Ct Cat C) U: ._ Ct m o ._ ._ C) 5 Ct C) C) ~. ~ _ L 2 Ad, ~ ~, D ~ ~7 2 _ ~o ~JO ~oo o ~o o ~ ~ ~.. ~Go _ ~ c`) ~ .= _ I, ^ o 3 _ o 3 , ~ Z O o ~ ~ - ~ - ~ ct ~ o ~, ,, '' C C9 3 3 Z Z 0 ~ e _, E ~ e - _ 3 3 5 ~ 3 G ~ ~ ~ _ - E S 3 0 ~c ~3 3 - -- o D > _ _ O - o ~ ~ D ~ V e _ _ ~ _ ,, ~O ~5\ A ~

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71 c ~ c ~ ~ 3 2 ~ c ~ ~ , ~ _ ~ c ~ z E ~ _ c i ~ e ~ _ c -_ 5 V C 0 ~ - ~ 3 ~ 0 0 0 3 _ c D - _ D ~ ~ ~ ~ ~ ~ O ~ ~ ~ ~ Q ~ = ~ ~ ~ = O O ~ ~ = O ~ ~ ~ ~ o ~ v, ~ 3 ~ ~ ~ ~ ~ ~ ~o ~ ~ 3 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ O ~Z _ ~00 ~Oo O ~ O O ~O O -O ,-_ ,<) t_ O u ~ ~ i c` .. 3 ~3 c e ~ a D ~e E 0 A ' e- , ~Z O ~Z 0 0 ~Z 5 3 5 C~ ~ ~ ~ -~ ~L)^ ~ ~ ,~ C) C Ct ~O e . _ ~ - G e, ~ 0 . c c 3 ~ E O ~ ~ E 3 c _ 3_ 3 E 3 > = D 3 e c ~ e c ~ s 3 c ~ ^, ~ r I - a v 3 ~ ~ ~_ ~ ~ ' c s .= _

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72 ._ 4 - o l Cal o ._ 2 Ct o -C5 Ct V, O ~ .= ~C: o ._ o us .o o U' so ~ Ct Cal U. ._ C,7 Ct o ._ .= Cal :: Cal ~o~ C;; ~ U. ~ , o i,~ ~ ' ~ i,, ~ 5 > Cal ~ AS Ad, .^ .= Ce o ~ ;^ 3 o ~ ~ C Ct _ ~ ~ ~ A= 3 a, 0 o ~ 3 ~ o _ 3 o _' ~ . _ Ct ~ ~ o of ~ ~3 ~t c,, o . X ~ .~- V, ~ ~ ~ - 0 =^ o ~C . X ~ 3 0 ~ 0 0 Z m ~^ 0 3 ~ ~`,, O <,4 C c 3 cy a a C - g e ~ v Y- _ _ B E c D D ~ c ~3 3 ~3 ~a . 0 OC' ~'= ~ ~ . ' r ~:, ~ ~ ~ C D ~ ~ ~ _ ~ 2 ~ C ;^ >` C~ ~ ~ Ct c: ~ Ct ~ Ct ~ C~ C~ C~ Ct . ~ - O ~ ,:D t4 D .- Ct ._ Ct ~ Z ~ 11 ~ .o Z_ . ct ~ ._ _ 0 . -,3 D ,- Ct ^^= ~ C O s._ O ~ =^ ~ ~X O ~ ~ ~2 . .- ._ ~ c~ ~ 2 s &-~ == ~ C~ ~ ~ . _ ~ Ct o o = o o ~ ~ _ ~ _ ~ ~ o Ct ~ o 11 _ ~ ~ ~ V) ~ C 11 .o 11 c, ~ ~ ,= 3 ~ Z 0 ~ ~ - 4 5 X `: O ~ Z g Z - 0 cr~

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ASSESSMENT OF GESTATIONAL WEIGHT GAIN 28 26 24 c', 1 8 7:s 1 6 O 14 12 10 8 6 4 - 1 I~ 1 1 1 1 1 ~ 1 1 1 1 1 1 1 1 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 Week of Gestation FIGURE 4-1 Weight gain god from Tompkins and W~ehl (1951~. 73 (Figure 4-5~. Several states (e.g., Indiana, Kansas, Mississippi, and Col- orado) use weight gain charts based on normative data for a group of low-income pregnant women from Cleveland and Minneapolis (Brown et al., 1986) (Figure 4-6~. For these charts, the total target weight gain for women with a normal prepregnancy weight is 14.3 kg (31.5 lb), which is approximately 2.9 kg (6.5 lb) higher than specified on most of the charts mentioned above. Several authors (e.g., Gueri et al., 1982; Husaini et al., 1986; and Rosso, 1985) have developed single charts or tables to be applied across a range of prepregnancy weights, heights, or weights for height. A table of reference weights for height developed by Gueri et al. (1982) is intended to be used at any stage of pregnancy. Figure 4-7 is based on data from that table to illustrate the weight gain patterns suggested by Gueri et al. The basis for and limitations of this table are discussed later in this chapter. In order to accommodate inconsistency in the application of propor- tionate weight gain recommendations across the range of prepregnancy weight for height, Rosso (1985) developed a weight gain grid that ap- plies different assumptions of weight gain patterns for underweight and overweight women (Figure 4-8~. Rosso's grid suggests that normal-weight women should be advised to achieve 120% of their prepregnancy refer- ence weight for height but that overweight women (e.g., those weighing up to 140% of reference weight for height) should gain up to 115% of their pr~pregnancy reference weight. Rosso's chart, like that of Gueri et al. (1982), suggests that underweight women gain all their weight deficit up to their prepregnancy reference weight as well as the additional weight

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85 o - ~ - 'e ~ ~ .= c ~ ~ c o ~ ~ ~ ~c,, ct o C C;~ C,) {_ C; ~3.~ _ _ :t c~ ~ ~ c< ~ vat ,~ D _ D ~ ,V=' ~ en ,= ~ D u: S O it, ,, ~ 'a E ~ E ~ ~ 5 ~ cot 3 . ~cat . ~_ cat _ _ _ y O O ~ - ' 5 _ ' c c ~ = = ,,0 2 = ~: c ~: c c 3 0 ~: ~ := c 3 ~ ~ c 3 E ~ ~ e ~ ~ 0 ~D ~ V ~ o u ~ ~ ' ~ O C ~ ed ~ ~ C 3 e a ~ 2 ' c c' ce 3 Z Z x - % - ~ - X D ~5 tV . 5 E ~ -= ~V ~ ~ ~ 8 -~ 5 _ 3 s 3 ~ ~ ~ ~'t O e ~ ~ 8 ' 0 c E E $ ~ 2 ~,8 x 2 8 ~u~ o ._ 3 ._ Ct o o ._ C~ oc ~S ._ - Ct s: o ._ Ct U. O4 ~o :^ oc Ct C~ ._ c: ~S ~o U' .= Ct

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86 NUTRITIONAL STATUS AND WEIGHT GAIN Estimation of Gestational Weight Cain in Research Information on gestational weight gain is commonly used in biomedical research, public health, and nutritional surveillance as well as in clinical care. Each application has its own requirements for the method of ex- pressing weight gain, but each benefits from rigorous attention to collecting high-quality data. The following discussion deals primarily with uses of gestational weight gain in research, which has stringent requirements for data quality and completeness. By far the most common expression of gestational weight gain in research is total weight gain based on the difference between an initial weight and a final weight taken in the last few weeks prior to delivery (Method 1 in Bible 4-3~. Rarely is it corrected for the weight of the fetus, placenta, and amniotic fluid in an effort to assess net weight gain or maternal tissue change (Method 2~. Occasionally, total weight gain is corrected for gestational length to yield a rate of gain (Method 3~. Increasingly, both researchers and clinicians are expressing gestational weight gain relative to either the observed or desirable prepregnancy weight (Method 4) or some recommended weight gain (Method 5), in which case the reference body weight (actual or desirable) should be expressed relative to maternal stature. All the methods of expressing gestational weight gain shown in Bible 4-3 have been used in research. A premium is placed on reliable weight measurements, accurate estimates of gestational age, and valid reference standards. Total weight gain based on recalled prepregnancy weight without correction for length of gestation has severe limitations. This is particularly true in the study of mechanisms by which weight gain and, more specifically, changes in maternal tissue and nutritional status affect maternal and fetal well-being. Measures of rate of weight gain, especially for specific phases of gestation, provide insight into the timing of intrauterine insults and, perhaps, into the mechanisms of fetal growth retardation and pregnancy and postpartum complications. However, the data needed to compute weight gain by stages of gestation are difficult to collect with adequate quality in all but the best-controlled prospective studies. Prepregnancy Weight Estimates Recalled prepregnancy weights may be acceptable for research under many circumstances (Stevens-Simon et al., 1986~; however, U.S. women of reproductive age tend to report current weights approximately 1.4 kg (3 lb) lower than their actual weights. Moreover, U.S. women who perceive themselves as overweight or who have less than a high school education underestimate their weight to a greater extent than do those who believe

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ASSESSMENT OF GESTATIONAL WEIGHT GAIN 87 their weights are normal or who have more formal education (Stewart, 1982~. Unlike weight, height tends to be overreported by an average of 1.1 cm (0.42 in.), especially among short women with limited formal education (Stewart, 1982~. Considering the negative bias in recalled weight and the positive bias in self-reported height, BMIs based on self-reported rather than measured prepregnancy weights and heights tend to be underestimated to a greater degree than Is weight alone. Weights determined at the first prenatal visit during the first trimester of pregnancy have been used to estimate total weight gain and early- gestation weight gain, but they do not necessarily reflect prepregnancy weights. Although average weight gain in the first trimester is small rel- ative to that in later periods of pregnancy, individual variation may be considerable. Total gestational weight gains may be overestimated by self- reporting or underestimated if based on late first trimester weight. Gestational Duration Estimates The use of reported LMP to estimate gestational duration has been shown (Kramer et al., 1988) to result in substantial misclassification of preterm and postterm (>42 weeks of gestation) births and thus, possi- bly, incorrect diagnosis of intrauterine growth retardation. This error in gestational age could cause an overestimate of the rate of weight gain in women delivering at full-term but whose LMP dates suggest a preterm birth. Kramer et al. suggest an even greater error in overestimating gesta- tional age among women with postterm LMP dates. Women who deliver their infants at full-term rather than postterm (as estimated from their LMP) may have an underestimate of their true rate of gain when that rate is calculated based on the LMP gestational age. 1b improve upon estimates of gestational length based on LMP dates, several methods have been used during pregnancy (sonography, symphysis- uterine fundal height) and at delivery (newborn maturity indicators). All these methods are based on measurements that are dependent upon length of gestation. In all cases, the estimates of gestational age were validated against gestational duration based on LMP, which is itself an imperfect measure. Moreover, all these techniques are based on measurements of fetal development that are also influenced by intrauterine environmental factors. Therefore, a 20-week-old fetus (whose age was estimated by sonography) who is determined to be growth retarded to the size of a 16-week-old fetus will be given a gestational age of 16 weeks, and the gestational weight gain of the mother may be charted at 16, not 20, weeks from conception. If there is an association between gestational weight gain and fetal growth, this relationship might be obscured by the fact that both do.

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g8 NUTRITIONAL STATUS AND WEIGHT GAIN the dependent variable (fetal weight or birth weight for gestational age) and the independent variable (weight gain) are biased in the same direction by errors in estimating gestational age. Determining Final Weight and Rates of Weight Gain Rarely is the final maternal weight measured at admission to the hospital just before delivery, but rather, it is measured at the last prenatal visit. Moreover, weights taken at the hospital vary according to the status of the membranes, i.e., whether or not they have ruptured, the content of the bladder and bowel, and the amount of clothing worn during the weighing, which is likely to have been done in a hurry. An alternative to using total weight gain is to compute a rate of weight gain up to the date of the last prenatal visit. Of course, any estimate of a rate of weight gain based only on initial and final weights assumes a linear pattern, which is not likely to be the case. Most evidence suggests that the lowest rates of weight gain occur during the earliest and latest weeks of a term pregnancy (see Chapter 5), when the frequency of the weighings may be most variable. A nonlinear pattern of weight gain plus the irregular time between weighings could result in more error in estimating the rate of weight gain as opposed to total weight gain. For example, a woman who gains a total of 11 kg (25 lb) between 12 and 37 weeks of gestation would have gained weight at a linear rate of 0.5 kg/week (1 lb/week). But if (as sometimes occurs) this initial weight is used as the prepregnancy weight (i.e., weight at week zero of gestation) and the weight at the last prenatal visit (e.g., at 37 weeks of gestation) is used as the delivery weight (e.g., at 39 weeks of gestation), then the computed rate is 0.3 kg/week (0.64 lb/week). More frequent weighings enable investigators to make more precise estimates of weight gain patterns according to the trimester of pregnancy, and would facilitate identification of the occurrence of fetal or maternal health problems by period of gestation. Including the Fetus in Total Weight Gain Total gestational weight gain includes the products of conception as well as maternal tissue (Method 1 in Table 4-3~. The definition may lead to problems of misinterpretation of studies relating maternal weight gain to birth weight. When the weight of the newborn is included in both the dependent and independent variables, a statistical situation known as part-whole correlation occurs. This is of considerable concern in research on maternal factors relating to variation in fetal growth when causal mech- anisms are being investigated (see Chapter 8~. In clinical settings, however, where total weight gain is used as a screening tool to assess fetal well-being,

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ASSESSMENT OF GESTATIONAL WEIGHT GAIN 89 it is advantageous that maternal weight gain reflects fetal size, especially if the screening takes place at a stage of pregnancy that allows intervention. ~ estimate maternal tissue mass more closely, researchers generally subtract the infant's birth weight from the total weight gain. At times, this procedure is refined by subtracting placental weight and an estimate of amniotic fluid weight. Or, when the data are available, net weight gain may be estimated by subtracting prepregnancy weight from postpartum weight obtained after delivery. Comparison of Gestaitonal Weight Gain with Reference Standards The expression of maternal weight or weight gain relative to a reference standard is especially useful if initial weights were taken after the first trimester of pregnancy and total or cumulative weight gain could not be calculated. One approach is to express weight at a given gestational age relative to prepregnancy body weight or desirable body weight (Method 4 in Table 4-3~. This is based on the assumption that weight gain should be proportional to either initial or desirable body weight. These two ways of expressing the reference prepregnancy weight can yield very different results at the extremes of the observed body weight distribution in a population. An overweight woman who is 165-cm (65-in.) tall and whose prepregnancy weight was 80 kg (176 lb) serves as an example. If she gains 15 kg (33 lb) during a pregnancy, she achieves 119% of her actual prepregnancy weight (95 kg/80 kg) but 161% of her "ideal" prepregnancy weight (i.e., 59 kg, according to 1959 MLI tables). On the other hand, a woman of the same height who weighs 45 kg (99 lb) and gains 15 kg (33 lb) during pregnancy would achieve 133% of her observed prepregnancy weight and 102% of her ideal weight. One advantage to the use of ideal prepregnancy weight as a reference weight is that it does not require knowledge of actual prepregnancy weight. Also, any analysis that includes weight gain expressed as a percentage of observed prepregnancy weight will be confounded by prepregnancy weight, the very factor that is independently related to pregnancy outcome and that may modify the effect of weight gain on the outcome. Using ideal body weight as the reference weight from which total pre- scribed weight gain is estimated, Gueri et al. (1982) presented a rationale for recommending a total gestational weight gain equal to 20% of the mother's ideal weight for height. They based their weight gain recommen- dations on four assumptions: (1) the desirable prepregnancy weights for height used are the average weights for U.S. women of different heights, as obtained for women between 18 and 24 years of age in the NHANES I survey, (2) the average total gestational weight gain should be 12 kg (26.4 lb), (3) the average increment of gestational weight gain should be 20% of

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go NUTRITIONAL STATUS AND WEIGHT GAIN the ideal prepregnancy weight, and (4) the average gestational weight gain of 1 kg (2.2 lb) in the first trimester and the gain of 11 kg (24.2 lb) in the next two trimesters follow a linear pattern. As illustrated in Figure 4-7, the target maternal weight at any gesta- tional age differs according to maternal height. A total gain of 12 kg (26.5 lb) equals 20% of the average weight of U.S. women of average height (163 cm, or 64 in.) in the reference population. Ibller women have a higher desirable body weight and would be expected to gain proportionately more than 12 kg. Similarly, shorter women would be expected to gain propor- tionately less weight. The adjustments for weight gain by trimester made by Gueri and colleagues were based on the assumption that regardless of the total prescribed weight gain, one-twelfth (8.3%) of the total gain should occur in the first trimester, and the remaining eleven-twelfths of the total gain should be divided equally between the remaining two trimesters (i.e., 45.8% of the total gain in each trimester). The application of this recommendation to women at the extremes of the weight-for-height spectrum poses a problem. Underweight women would be expected not only to compensate for their entire deficit in prepreg- nancy body weight but also to add the tissues of pregnancy during a 9-month period. For a 160-cm (63-in.) tall woman whose prepregnancy weight was 47.5 kg (104 lb), this would amount to a gain of 23.5 kg (52 lb) 50% of her prepregnancy weight-during the course of the pregnancy. The subcommittee found no evidence that such a large weight gain would be desirable for underweight women. Another serious problem with the table of Gueri et al. occurs at the upper end of the range of prepregnancy weight for height. Women at 120% of the reference weight would be advised to gain no weight during pregnancy, and those over 120% of the reference weight would be advised to lose weight during pregnancy. Husaini et al. (1986) developed a weight gain grid for Indonesian women based on the same principles used by Gueri and colleagues, but they used observed weight gain among women who delivered infants weighing more than 2,500 g (5.5 lb) at birth. Overweight was not a factor in this poor, rural Javanese population, so their patterns of weight gain for different heights appear to follow those presented by Gueri et al. However, the achieved weights at any given height and at any given week of gestation are considerably lower than those suggested by Gueri and associates. Furthermore, the weight gain chart for Indonesian women does not assume a proportionate weight gain; short women of any prepregnangy weight would gain the same absolute amount of weight during gestation as taller women of the same relative weight for height. Another modification of this approach is to express weight gain as a percentage of recommended gain, either over the entire pregnancy or over specified periods (Method 5 in Table 4-3~. This requires the same

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ASSESSMENT OF GESTATIONAL WEIGHT GAIN 91 measurements as in Methods 1 and 3 but expresses the resulting gains relative to a standard. This is similar to the way one expresses weight or height in children relative to a postnatal growth standard. The advantage of this method is that it allows weights collected at different frequencies during different stages of pregnancy to be compared on a similar scale (as percentages of reference weight gain) and thus removes the time depen- dence of weight changes described above. This is also the preferred way of expressing gestational weight gain when studying its effect on length of gestation or risk of premature delivery. Since premature delivery will shorten the time over which weight gain occurs, there is a need to ex- press weight gain in a way that is independent of the length of gestation. Many researchers choose to express weight gain as a rate. However, this approach usually does not account for the nonlinear nature of weight gain throughout gestation, which will result in a bias toward underestimating the true rate of weight gain if the pregnancy is terminated early. This bias could lead to an observed negative relationship between rate of weight gain and risk of preterm delivery that may not actually exist. One solution to this problem is to express weight gain as a percentage of a reference weight for a given week of gestation. Of course, the method is valid only if the correct standard is used and if the percentage deviation from the reference curve is interpreted similarly at different weeks of gestation. Given current knowledge of the use of this approach to evaluate early postnatal growth, it is likely that a given percentage of reference maternal weight for gestational age at 20 weeks of gestation implies a different degree of risk than the same percentage of reference weight at 35 weeks of gestation (Haas and Habicht, 1990~. At the very least, the method requires knowing if the coefficient of variation (standard deviationJmean) of weight gain in normal pregnancies with favorable outcomes is constant over all gestational ages. If it is not constant, then a given percent deviation from a mean weight at a given week of gestation will not have the same percentile standing as it would at another week of gestation. USES OF GESTATIONAL WEIGHT GAIN IN NUTRITIONAL SURVEILLANCE In recent years, a system of nutritional surveillance has been under development in the United States. The major objective of such a system is to provide accurate information to decision makers who are responsible for policy, program development, and management in areas that affect nutrition and public health (DHHS/USDA, 1986; Mason et al., 1984~. The information derived from nutritional surveillance can also be of use in clinical practice (Won" and Footbridge, 1984~. The Centers for Disease Control have been charged with the coordination of nutrition surveillance

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92 NUTRITIONAL STATUS AND WEIGHT GAIN activities, which include identifying data sources, standardizing data col- lection instruments, training people in data collection, conducting data analysis, and disseminating information to appropriate users. Two compo- nents of nutritional surveillance related to the work of this subcommittee are the Pregnancy Risk Assessment Monitoring System and the Pregnancy Nutrition Surveillance System. The goal of these two systems is to con- tribute to the reduction of pregnancy-related health risk factors and adverse pregnancy outcomes in both the general population and the low-income population of the United States. In these surveillance systems, gestational weight gain is included as a maternal measure associated with health risk and pregnancy outcome. The extensive standardized data collection instrument developed for these two systems includes questions on prepregnancy weight and height as well as total gestational weight gain and length of gestation (from LMP). There appears to be no information on patterns of weight gain or a specific identification code for the sources of data on weight and height (measured versus recalled). Since these systems are still in the development stage, it has not been possible to evaluate how these data are to be analyzed and whether some of the concerns raised elsewhere in this chapter have been addressed. The analytical depth and sophistication often required of scientific research cannot be applied to nutritional surveillance data. Reports must be generated in a timely fashion if they are to be of use to decision makers, and a sophisticated statistical analysis may not be understood by many of the potential users. However, most of the concerns regarding data quality and interpretation raised under research applications also apply to use of gestational weight gain data in nutritional surveillance. Operation of the two relevant surveillance systems may produce ben- efits in addition to those previously stated. Because of increased efforts to standardize data collection methods among the many state programs, there is likely to be an increase in the amount and quality of data available on gestational weight gain, its antecedents, and its consequences, so that important research can progress. Also, this standardization process may extend to clinical uses of gestational weight gain charts such as those found in individual state WIC programs. SUMMARY Clinical practice, research, and surveillance activities benefit from the implementation of proper measurement techniques. Interpretation of results from studies of gestational weight gain requires careful attention to definitions of gestational weight gain and gestational duration.

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ASSESSMENT OF GESTATIONAL WEIGHT GAIN CLINICAL IMPLICATIONS 93 Prepregnancy weight is an important factor. Objective data, such as those obtained from a medical record, are preferred. Information provided by the patient should be evaluated for its accuracy. Height without shoes should be determined at the first prenatal visit, preferably with a wall stadiometer whose accuracy has been verified. Gestational age should be estimated from the date of onset of the woman's last menstruation, preferably supplemented by estimates based on the obstetric clinical examination and perhaps by early ultrasound. A weight-for-height category is derived from the patient's height and prepregnancy weight. The resulting BMI can be compared to the recommended classification in Appendix C. This comparison will provide a foundation for specific nutrition counseling and the creation of a plan for overall and incremental weight gain. The patient's weight should be obtained at the beginning of each prenatal visit. Consistently, the patient should be weighed without her purse and outdoor clothing. If weight cannot be obtained routinely without shoes, a marked variation in the type of footwear should be noted. . The immediate recording of measurements directly on an appro- priate form is encouraged. Recording data on a chart that depicts weight gain by gestational age provides a visual impression of sequential changes. Abrupt or inconsis- tent changes should be scrutinized to determine whether they reflect errors or accurate data. Members of the clinical staff should be trained in proper measure- ment techniques. Their performance should be monitored periodically. Equipment for taking measurements should be calibrated periodi- cally. Only the patient's approximate weight is obtained at the time of measurement. An exact measurement is not possible because of clothing, contents of bladder and bowel, time of day, time of last meal, and other such factors. Therefore, interpretation of weight change should focus on sequential data or trends. The patient's weight should be compared with the target weights and rates of gains depicted in Chapter 1, Table 1-1, and Appendix B. Investigation to determine the cause of inappropriate patterns will permit specific remedial action to be taken. REFERENCES Brown, J.E., K.W. Berdan, P. Splett, M. Robinson, and L^J. Harris. 1986. Prenatal weight gains related to the birth of healthy-sized infants to low-income women. J. Am. Diet. Assoc. 86:1679-1683.

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94 NUTRITIONAL STATUS AND WEIGHT GAIN Burton, B.T., W.R. Foster, J. Hirsch, and T.B. Van Itallie. 1985. Health implications of obesity: an NIH Concensus Development Conference. Int. J. Obesity 9:155-170. Butman, M., ed. 1982 Prenatal Nutrition: A Clinical Manual. WIC Program, Massachusetts Department of Public Health, Boston, Mass. 158 pp. DHHS/USDA (I)epartment of Health and Human Sewices/U.S. Department of Agriculture). 1986. Nutrition Monitoring in the United States: A Progress Report from the Joint Nutrition Monitonng Evaluation Committee. DHHS Publ. No. (PHS) 86-1255. Public Health Service. U.S. Government Printing Office, Washington, D.C 356 pp. Limpers, D. 1988. Prenatal Nutrition: Clinical Guidelines for Nurses. March of Dimes Birth Defects Foundation, White Plains, N.Y. 134 pp. Flegal, KM., W.R. Harlan, and J.R. Landis. 1988. Secular trends in body mass index and skinfold thickness with socioeconomic factor in young adult women. Am. J. Clin. Nutr. 48:535-543. Georgia Dietetic Association. 1987. Diet Manual of the Georgia Dietetic Association, 3rd ed. Georgia Dietetic Association, Duluth, Ga. 441 pp. Grifflths, M. 1985. Growth Monitoring of Preschool Children: Practical Considerations for Primary Health Care Projects. World Federation of Public Health Associations, Washington, D.C. 79 pp. Gueri, M., P. Jutsum, and B. Sorhaindo. 1982. Anthropometric assessment of nutritional status in pregnant women: a reference table of weight-for-height by week of pregnancy. Am. J. Clin. Nutr. 35:609-616. Haas, J.D., and J.P. Habicht. 1990. Growth and growth charts in the assessment of preschool nutritional status. Pp. 160-183 in G.A. Harrison and J.C. Waterlow, eds. Diet and Disease in Traditional and Developing Societies. Cambridge University Press, Cambridge. Hamill, P.V.V., T.A. Dnzd, C.L. Johnson, R.B. Reed, and AF. Roche. 1977. NCHS Growth Curves for Children from Birth to 18 Years: United States. Vital and Health Statistics, Series 11, No. 165. DHHS Publ. No. (PHS) 78-1650. National Center for Health Statistics, Public Health Service, U.S. Department of Health, Education, and Welfare, Hyattsville, Md. 74 pp. Husaini, Y.K., M.A. Husaini, Z. Sulailuan, A B. Jahari, Barizi, S.T. Hudono, and D. Karyadi. 1986. Maternal malnutrition, outcome of pregnancy, and a simple tool to identify women at risk. Food Nutr. Bull. 8:71-76. Hytten, F.E., and I. Leitch. 1971. The Physiology of Human Pregnancy, 2nd ed. Blackwell Scientific Publications, Oxford. 599 pp. Idaho WIC Program. 1988. Idaho WIC Program Procedure Manual: Special Supplemental Food Program for Women, Infants, & Children. Bureau of Maternal and Child Health, Division of Health, Department of Health and Welfare, State of Idaho, Boise, Idaho. (various pagings). Jackson, AS., M.L Pollock, J.E. Graves, and M.T. Makar. 1988. Reliability and validity of bioelectrical impedance in determining body composition. J. Appl. Physiol. 64:529-534. Kleinman, J.C. 1990. Maternal Weight Gain During Pregnancy Determinants and Conse- quences. NCHS Working Paper Series No. 33. National Center for Health Statistics, Public Health Service, U.S. Department of Health and Human Services, Hyattsville, Md. 24 pp. Kramer, M.S., F.H. McLean, M.E. Boyd, and R.H. Usher. 1988. Ibe validity of gestational age estimation by menstrual dating in term, preterm, and postterm gestations. J. Am. Med. Assoc. 260:3306 3308. Lohman, T.G., Am. Roche, and R. Martorell, eds. 1988. Anthropometric Standardization Refererce Manual. Human Kinetics Books, Champaign, Ill. 177 pp. Lull, C.B., and R.A. Kimbrough, eds. 1953. Clinical Obstetrics. Lippincott, Philadelphia. 732 pp. Mason, J.B., J.P. Habicht, H. Tabatabai, and V. Valverde. 1984. Nutritional Surveillance. World Health Organization, Geneva. 194 pp. Metropolitan Life Insurance Company. 1959. New weight standards for men and women. Stat. Bull. Metrop. Life Insur. Co. 40:1-4. Metropolitan Life Insurance Company. 1983. 1983 Metropolitan height and weight tables. Stat. Bull. Metrop. Life Found. 64:3-9.

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