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4 addition to prolonged hospitalization at birth, almost 40 percent of very Tow birthweight infants are rehospitalized more than once during the first year, for an average of 16 days. This compares with 19 percent of all Tow birth- weight infants for an average of 12.5 clays, ant! 8.7 percent of normal birth- weight infants for an average of ~ clays. The birth of a low birthweight infant and the problems that follow may place substantial emotional and financial stress on a young family. The effects of this stress on the well-being of the infant, on the parent-chiTc! bond, and on siblings and the marital relationship are still being investigated. Decreasing Mortality: The Effect on Morbidity Greater success in saving the lives of low birthweight infants has not increased our burclen of babies with handicaps, according to recent studies. The proportion with severe congenital malformations or retarded develop- ment remains the same, and there has been a decrease in the proportion of those with less severe problems related to birth. Concern remains, however, about the effects of increased survival of the very smallest infants, those less than 1,000 grams. These survivors constitute a relatively new population that will require long-term follow-up and evaluation. CAUSES OF LOW BIRTHWEIGHT An analysis of the causes of Tow birthweight must differentiate between those responsible for premature birth and those leading to intrauterine growth retardation (TUGR). Trying to separate the two conditions is compli- cated, however, because {UGR and prematurity occur together in a sub- stantial portion of low birthweight cases. The physiological and biochemical events that initiate and maintain nor- mal human labor are not well understoocI, although recent investigations have producecl important new clues about hormonal factors in the process. Theoretical models based on these clues have allowed researchers to begin studying variations from the normal pattern that might lead to premature labor. Certain clinical conditions, discussed in the following section on risk factors, appear to cause changes in the hormonal environment and metabolic state of the uterus and cervix. These changes probably result from complex interactions involving progesterone, estrogen, oxytocin, and other hor- mones; prostaglandins; calcium ions; adrenergic agents and receptors; catecholamines; and uteroplacental blood flow. lUGR is associated with conditions that interfere with the blood circulation to and efficiency of the placenta, with the development of the fetus, or with the general health and nutrition of the pregnant woman. In many cases, however, no relevant pathogenic factors can be found. Maternal vascular diseases, such as chronic hypertension, chronic renal disease, or sickle-cell disease, may hamper delivery of nutrients or oxygen to the fetus. Multiple pregnancies may result in fUGR because the placenta cannot supply suffi- cient nutrients to more than one fetus. Fetal factors associated with JUGR include chromosomal disorders, chronic fetal infections such as congenital rubella and syphilis, and radiation injury. All of these associations suggest

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;;- d-~ I] - : - . - . - ~ Alp:: : :: . z i: A: - o - ~ - possible pathogenic mechanisms, but the underlying physiological pro- cesses have not been identified. Risk Factors In the absence of adequate information about the basic causes of low birthweight, a large body of information has cleveloped about "risk factors," or factors whose presence in an individual woman indicates an increased chance of bearing a low birthweight infant. These factors, which will be outlined further in the following pages, are listed in Table I. They include demographic characteristics, medical risks that can be iclentifiecT before preg- nancy and those that can only be identified during pregnancy, behavioral and environmental factors, risks associated with health care (such as inade- quate prenatal care), and a separate group of factors whose relationship to low birthweight is more tenuous, such as stress, uterine irritability, and inadequate plasma volume expansion. Grouping the risk factors as noted on the table helped the committee identify those that can be detected before pregnancy and reinforced the concept that interventions can begin before the prenatal period. Smoking is perhaps the best example of this perspective. The grouping also emphasizes the importance of behavioral and environmental risks and the need for interventions that go beyond medical care. The demographic measures can help to define target populations. The cluster of health care factors highlights the fact that not all risks for low birthweight derive from characteristics of

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6 women themselves. And finally, the category of evolving concepts of risk suggests some important research areas. These themes appear throughout the main report. The committee concluded that a variety of factors are clearly and con- sistently linked to low birthweight. These factors should be used to help define high-risk groups and to develop and target interventions. It is appar- ent, however, that the magnitude of risk posed by each factor for an individ- ual or for a group cannot always be calculated easily, that the risks for Tow birthweight are widely distributed throughout the population, and that a substantial number of low birthweight cleliveries will continue to occur outside of groups currently clefined as high risk. These circumstances high- light the need for greater understanding of risk and causation, but should not be used to minimize the value of using existing risk information for targeting interventions. Demographic Risks Because the major demographic risk factors are often interrelated, it has been difficult to determine the precise association between any single factor and low birthweight. Nonetheless, careful research is graclually defining the independent effects of many factors. Race Black newborns are more than twice as likely to weigh less than 2,500 grams as white infants. The race-specific low birthweight rates among live births in the United States in 1981 were 12.5 percent for blacks and 5.7 percent for whites. The reasons for the higher risk among blacks are not clear. It has been speculatecl that maternal age may account for part of the difference twice as many black births are to teenagers but when black and white mothers of the same age are compared, blacks are at higher risk of Tow birthweight in every age group. Similarly, black mothers as a group have less education that white mothers, but when blacks and whites are matched by level of education, blacks still have a higher risk of low birthweight. Other factors that have been stuclied but fad! to account for the white-black differen- tial inclucle delay in initiating prenatal care, smoking status, height and weight distributions of the mother, and obstetric history. The committee's analysis of national and statewide trends in the white- black differential in low birthweight indicates that the gap is not closing. For the United States as a whole, the relative decline in white Tow birthweight rates between 1971 and 1981 exceeded the corresponding relative decline in black low birthweight rates. White Tow birthweight rates cleclined by 14 percent between 1971 and 1981, while black rates declined by only 6 percent. The absolute declines among whites and blacks, however, were more com- parable. It appears, therefore, that characterizing trends in the difference between black and white rates of Tow birthweight depends to some extent on the measures used. The issue of race and Tow birthweight is further complicated by the differ- ent birthweight-specific neonatal mortality rates of white and black infants. Black infants weighing less than 2,500 grams have Tong been recognized to have better rates of survival in the neonatal period than white infants of

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7 TABLE 1 Principal Risk Factors for Low Birthweight I. DEMOGRAPHIC RISKS A. Age (less than 17; over 34) B. Race (black) C. Low socioeconomic status D. Unmarried E. Low level of education II. MEDICAL RISKS PREDATING PREGNANCY A. Parity (0 or more than 4) B. Low weight for height C. Genitourinary anomalies/surgery D. Selected diseases such as diabetes, chronic hypertension E. Nonimmune status for selected infections such as rubella F. Poor obstetric history including previous low birthweight infant, multiple spontaneous abortions G. Maternal genetic factors (such as low maternal weight at own birth) III. MEDICAL RISKS IN CURRENT PREGNANCY A. Multiple pregnancy B. Poor weight gain C. Short interpregnancy interval D. Hypotension E. Hypertension/preeclampsia/toxemia F. Selected infections such as symptomatic bacteriuria, rubella, and cytomegalovirus G. 1st or 2nd trimester bleeding H. Placental problems such as placenta previa, abruptio placentae I. Hyperemesis I. Oligohydramnios/polyhydramnios K. Anemia/abnormal hemoglobin L. Isoimunization M. Fetal anomalies N. Incompetent cervix O. Spontaneous premature rupture of membranes IV. BEHAVIORAL AND ENVIRONMENTAL RISKS A. Smoking B. Poor nutritional status C. Alcohol and other substance abuse D. DES exposure and other toxic exposures, including occupational hazards E. High altitude V. HEALTH CARE RISKS A. Absent or inadequate prenatal care B. Iatrogenic prematurity VI. EVOLVING CONCEPTS OF RISK A. Stress, physical and psychosocial B. Uterine irritability C. Events triggering uterine contractions D. Cervical changes detected before onset of labor E. Selected infections such as mycoplasma and Chlamydia trachomatis F. Inadequate plasma volume expansion G. Progesterone deficiency similar birthweight. Based on this observation, some researchers have sug- gested that the 2,500-gram marker of low birthweight may not have the same implications for nonwhite infants as for whites; however, this line of reason- ing is overshadowed by the more imposing fact that nonwhite infants are twice as likely to be born at low birthweight and twice as likely to die in the neonatal period as white infants. The conclusion to be drawn from the complicated ciata on race, low birthweight, and race-specific birthweight mortality rates is that the reasons for the risk differentials between white and black newborns are not well understood. The cumulative effects over time of black poverty and lower social status, and the interaction of such factors with biological processes, undoubtedly have played a role in these racial differences; other factors remain to be defined. Research should be pursuer! to improve our under- standing of these important issues.

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8 Age Teenage mothers and those age 35 years or older have higher rates of low birthweight than mothers in their twenties and early thirties. The risk is highest for young mothers, especially among whites; however, childbearing in the teen years is more prevalent among blacks. In 1980, 211 percent of all black births were to teenagers, compared with 12.! percent of white births and 15.3 percent of Hispanic births. Studies indicate that young age is not an independent risk factor for Tow birthweight. Teenage mothers have many other characteristics that increase the likelihood of a low-weight birth. They are more likely to be black, of low socioeconomic status, and unmarried than older mothers. Also, they are shorter and lighter, less educated, and more likely to report late for prenatal care than their older counterparts. Socioeconomic Status Tow socioeconomic status (SES), measured in terms of social class, income, education, or census tract, is clearly associated with an increased risk of preterm delivery and TUGR. The literature suggests that at least some of the excess risk arises from separate factors linked both to low social class and low birthweight. These include smoking, low maternal weight gain and short stature, obstetric complications such as hypertension and preecIampsia (a toxic condition of late pregnancy), some types of genitourinary tract infections, and limited access to high-quality prenatal care. The effect of socioeconomic status probably represents the sum of many factors, each of which may increase the risk of poor pregnancy outcomes. Education The risk of low birthweight declines sharply among mothers with at least 12 years of education. The relationship between education and low birthweight is independent of maternal age and race. The committee's analysis of national data indicates that the gap in Tow birthweight rates among mothers with different levels of education is not closing and may be widening. This finding is especially important given that the educational attainment of mothers has increased significantly during the past 10 to 15 years. The widening gap suggests that the poorly educated may constitute an increasingly high-risk group. Marital Status Unmarried mothers have a consistently higher risk of bearing a low birthweight infant than those who are married. This risk is not attributable to differences in age or race. In 1980, the Tow birthweight rate for infants born to unmarried mothers was Il.6 percent, compared with 5.8 percent for babies whose mothers were married. The significance of marital status as a risk factor is underscored by the increase in childbearing among unmarried women. Between 1976 and 1981, the proportion of white mothers reported to be unmarried increased from 7 percent to 12 percent; for blacks, the proportion increased from 51 percent to 56 percent. Medical and Obstetric Risks Medical and obstetric risks for low birthweight can be divided between those detectable before pregnancy, such as chronic illness in the mother or a history of poor pregnancy outcome, and those that can be noted only during pregnancy, such as placenta previa. The committee focused on a subset of

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9 these problems: hypertension/preecIampsia, diabetes, obstetric history (in- clud~ng previous Induced abortion), multiple pregnancy, and infection. Hypertens ion/PreecZa mps ia Hypertension is the disease most often associ- ated with fetal growth retardation; it also can be associated with preterm delivery. In one study population in the 1970s, researchers found that 27 percent of lUGR with an identifiable cause could be attributed to severe preecIampsia, chronic hypertensive vascular disease, or chronic renal dis- ease.2 Infants with TUGR were born to 30 percent of patients with a diagnosis of chronic hypertension and to 46 percent of patients with severe preecIamp- sia. Elevated maternal blood pressure also may cause preterm labor (often precipitated by premature detachment of the placenta), or necessitate medi- cal intervention to deliver the baby and thereby avoid more serious prob- lems. Diabetes Maternal diabetes mellitus is frequently associated with babies born large for gestational age, but the disease also may lead to JUGR or preterm delivery. In the past, diabetes-related premature births often re- sulted from physician interventions to avoid unexpected intrauterine death. These early deliveries were appropriate in some cases but not in others. Improved management of diabetes and new techniques to assess fetal well- being, gestational age, and lung maturity have reduced the number of unnecessary early deliveries associated with maternal diabetes. Premature delivery may still be necessary in some cases, however, for pregnant women with insulin-dependent diabetes complicated by diabetic vasculopathy. In such women, preterm delivery may be required because of worsening mater- nal retinopathy, nephropathy, or Hypertension. Increasing evidence indicates that poor control of maternal diabetes dur- ing the early weeks of embryonic development may contribute both to poor fetal growth and congenital defects. Researchers have shown that excellent control of diabetes before conception and during the early weeks of pregnan- cy can decrease the risk of fetal malformation,3 but the effects of such control on fetal growth have not been explored. Obstetric History The history of a woman previous pregnancies is of prime importance in the prediction of a subsequent Tow birthweight infant. A detailed study of the weights and gestational ages of all births in Norway from 1967 through 1973 showed that a premature first birth is the best predictor of a premature second birth and that growth r~t~r~l~tinn in ~ [irct . . _ _ O ~ % ~ . . ~ ~ ~ ~ ~7 . pregnancy IS the most powerful predictor of growth retardation in a second pregnancy.4 Previous fetal and neonatal deaths also are strongly associated with preterm low birthweight, and the risk increases as the number of poor fetal outcomes increases. The effect of the interaction between maternal age and birth order on Tow birthweight has been well documented. The incidence of low birthwei~ht is 1~ ~1_ r~ 1 . ~ _ ~ ~ ~ ~ Sign for women Between ~ ~ and ~v bearing their second or later child, Tow for women age 25 to 34 bearing their third or later child, and increases sharply among women having their first child after age 29. Special tabulations on 1981 live births performed for the committee by the National Center for Health Statistics show that interval between pregnancies

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10 also affects low birthweight. The risk is sharply elevated for an interval of less than 6 months, decreases moderately from 6 months to between 24 months and 36 months, and then rises gradually. However, short interpregnancy interval is not associated with an increased risk of Tow birthweight if the previous pregnancy ended in a fetal death. Previous Induced Abortion Because about 1,500,000 legal induced abor- tions occur annually in the United States, the committee believed it was important to assess the impact of such procedures on the incidence of {UGR and preterm delivery in subsequent pregnancies. A review of the literature showed that: the risk of Tow birthweight or preterm delivery in a pregnancy following one that is terminated by vacuum aspiration abortion in the first trimester (the most common abortion procedure in the United States) is no greater than the risk of adverse outcome expected for a first pregnancy; and the effect of multiple abortions on subsequent pregnancies is unclear; some studies have shown an increased risk of Tow birthweight and others have not the outcome may depend on the type of abortion procedure performed. There is some concern that abortion techniques requiring cervi- cal dilatation of more than 12 millimeters may lead to problems of cervical incompetence, and therefore an increased risk of prematurity in subsequent pregnancies. The committee concluded that research is needed to investigate further the relationship of induced abortion to the outcome of future pregnancies. MuZtipZe Pregnancy Pregnancies with twins, triplets, or more carry an increased risk both of preterm delivery and low birthweight. Even at full term, infants in plural deliveries are T! times more likely to be of Tow birthweight than singleton deliveries. Neonatal mortality also is greatly increased in multiple pregnancies, and morbidity is high among survivors. Infections A variety of infections have been linked with both preterm delivery and intrauterine growth retardation. For some infectious agents, a causal role in Tow birthweight has been established; for others, the associa- tion is less clear. Many of these infections can be prevented or treated, reducing the risk of an adverse pregnancy outcome. Congenital rubella syndrome and cytomegalovirus infection, both commonly associated with congenital defects, also can cause intra-uterine growth retardation. The incidence of rubella virus infection in the United States and the number of congenitally infected newborns have clecreased significantly since the introduction of the rubella vaccine. Untreated or inadequately treated symptomatic urinary tract infections are known to cause a variety of problems for both mother and fetus, including Tow birthweight. Pregnant women with no symptoms of such infections (asymptomatic) should be screened routinely for bacteriuria (the presence of bacteria in the urine), because symptomatic infections can be prevented in most patients by treatment of asymptomatic infections. Culture techniques are inexpensive anct easy to use; patients with known infections can be taught to culture their urine at home. Another organism that may leac! to Tow birthweight as a result of maternal

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11 genitourinary infection is mycoplasma. In one recent study, mycoplasma- infected women treated with a 6-week course of erythromycin showed a markedly recluced incidence of low birthweight.5 More research is needed to confirm this effect and to explore its significance. Some researchers believe that certain genital pathogens may trigger pre- term labor, as well as affect intrnl]tf~rino Growth To ~nlr~r~o this role_ I- _ _ _1 ~.1 ~ en ~ ^ ~ ~^ ~ ~ ~_~ ~ ~ ~ ~ ~ ~ V ~ ~1 L. 1 V ~A~lW1 ~ L1 [~= 1 Elm t~onsn~ps further, future studies must examine comprehensively the flora of the cervix and vagina, identify local and systemic immune responses, and assess their combined influence on pregnancy outcome. Nutrition Four types of research have been used to examine the effect of nutrition during pregnancy on birth outcomes: animal stuclies, human war famine studies, nutritional intake/fetal outcome correlational studies, and ex- perimental nutrition intervention studies. They all point to the common conclusion that good nutrition has a positive influence on birthweight, but the extent of the effect is unclear. The magnitude of nutritional effects on low birthweight is not easily assessed because nutritional status is difficult to isolate from other socioeconomic characteristics and because of the compli- cated relationship between prepregnant weight and weight gain during pregnancy. While researchers have found positive relationships between birthweight and nutritional status, there is wide variability in the degree of these associations. A reasonable conclusion is that poor nutritional status before pregnancy and inadequate nutrition during pregnancy have a nega- tive impact on fetal weight gain, thereby increasing the risk of JUGR. One recent study explored the relationship between a mother's weight gain during pregnancy and the occurrence of low birthweight by analyzing data from the 1980 National Natality and Fetal Mortality Surveys. The inves- tigators found that many groups of women known to have an increased risk of delivering a low birthweight infant also were more likely to have inade- quate weight gains. For example, they found that black mothers were twice as likely as white mothers to gain less than 16 pounds during pregnancy. In addition, mothers 35 years of age or older and teenage girls were less likely to gain at least 16 pounds, as were unmarried women, poorly educated women, anc! women of lower socioeconomic status. A further analysis of numerous risk factors among white mothers only indicated that, except for period of gestation, weight gain has the strongest impact on birthweight.6 Behavioral and Environmental Risks Smoking Smoking is one of the most important and preventable cletermi- nants of low birthweight in the United States. A recent survey of the litera- ture on smoking and birthweight indicates that smoking during pregnancy is associated with a reduction in birthweight ranging from 150 to 250 grams. This relationship has persisted for at least 20 years, despite reported reduc- tions in the average tar and nicotine yields of cigarettes on the market.7 The reasons for the detrimental effects of cigarette smoking are not fully under- stood, but the fact that an estimated 20 to 30 percent of pregnant women in the United States smoke underscores the importance of this risk factor.

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12 Alcohol Use The data on maternal alcohol consumption and its association with low birthweight are not as uniform as for smoking. it is reasonably certain that pregnant women who drink heavily are at risk of delivering a baby with fetal alcohol syndrome characterized by lUGR and a variety of other problems. The impact of moderate alcohol use is less clear. A 1983 review of the literature suggested that regular drinking of fewer than two drinks per day probably is not an important determinant of JUGR,7 but two more recent studies contradict this conclusion. In one, a prospective study of 30,000 pregnancies, women who drank one or two drinks per day proved to have an increased risk of {UGR even after the figures were adjusted for maternal age, race, education, marital status, and a variety of other risk factors.8 Uncertainty over the effects of alcohol on fetal development warrants caution: the Surgeon General of the United States has advised pregnant women not to drink alcoholic beverages. latrogenic Risks {atrogenic prematurity refers to the birth of a physiologically immature and/or low-weight infant who is delivered prematurely as a result of medical intervention. Some cases are justifiable-the decision to end a pregnancy early may be made to avert more serious consequences for the mother or infant but others are practitioners' mistakes. Studies conducted in the early and micI-1970s found that from 4 to ~ percent of infants admittect to neonatal intensive care units in three cities had been born prematurely as a result of labor inductions and electively timed cesarean sections.9~~ Accurate prenatal assessment of gestational age, combined with selected use of ultrasound examination anc! new techniques to test fetal lung matur- ity, could reduce the number of cases of accidental iatrogenic prematurity. {atrogenic prematurity also could be reduced by decreasing the number of primary and repeat cesarean sections. Evolving Concepts of Risk A desire to improve the health care professional's ability to identify preg- nant women at risk of a low-weight birth has lect researchers to study a variety of other possible risk factors. Those described in the full report include stress, uterine irritability anct the notion of "triggering factors," certain cervical changes cletected before labor begins, inadequate plasma volume expansion, and progesterone deficiency. The first three factors only are outlined below. Stress The relationship between socioeconomic status and Tow birth- weight suggests that a woman's response to her environment may have an impact on pregnancy outcome; it may be, for example, that poverty is a risk factor for low birthweight because of the high levels of stress associated with being poor. Two types of stress have been examined in numerous studies: physical stress and fatigue, particularly as related to employment during pregnancy, and psychological distress resulting from maternal attitudes toward the pregnancy or from external stressors in the environment. Major studies of physical stress (usually in a work-related setting) indicate

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13 that there may be some association between low birthweight (both {UGR anct prematurity) and activities that require long periods of standing or other significant amounts of physical stress. There is no conclusive evi- o dence, however, that maternal em- ~ ployment per se increases the risk of Tow birthweight. ,: Numerous reports suggest a link between stress and conditions that ~- increase the risk of JUGR and pre- x term labor, such as preecIampsia, but ~ relatively little ciata exist to support a O strong, direct relationship between maternal psychological stress and low birthweight. Some data link psy- chological stress to a number of other . . . to ~ ~ __. .... __ em _... _- ~ _...... _ ~ ..... ~ ~_. . 1 _.$ - .. 1~...' _. _ ~ $ : ~$~ _ ..... ..~ I, I, ,.,. :,~i X X, I I. :, :'.' _ ~ ., :.', .,'., " ~ .$. :,$ I: : ,. A :: .: . . $ic,,.,,., __ . ~ .'. . :,.. .,. . ~ :$..: ~ ' :. . A.:.: . . .:.'. .~.'$~":.:. _ $$ X$ $ ~ ^$ x ^$ $ x .$. .$.$$X. $.~ -.:.e $::::X~d'.'.$ . .$. $.... $....~$...- ....t _ eF ~ $~ $ ~ $ '$~$. ;''' _ ~ ~ XX $ ~ ~ x'$$' "' ~ 2-' - - ..'"''''.'.'''''''""'.""'2"''.'.2" _ - $ x$$ 4_ he _~=.] $] . X$ $ $ ~ : ~ I... .: ~ . : . ~ .. I. . . ..: :. ~ . . ~.::::~::#$. . . 111` . ~$~ x$ .:- -:.:.:. - .:. .:.:.:.:::: ::::::::: ::::::::::::: ::::::: :::: :::::: Y .::'::i,. R. . $ - : : :: :. . . . . . ::: .. :. . $q~ . . . . x $ . $_ ,::: :.:.:.:.:::: ::::::::::::: ::::::.:: ::::::. [.~ Ad: ::~:...~ ~ :::~.,:~:.:.,: .,: _ F:::,..= _= _X$$::::::::$$$$$$.:.: $ $. a., , ,: . :.:.:.:.:.:.:~i: i -::: _ X _.:,::::::::::::::::~_ |~ it= ~ $$" - "$X$' =;;;;; me_ ['~ poor pregnancy outcomes such as fetal distress, neonatal motor immaturity, perinatal deaths, and congenital anomalies. Research in this area is plagued by a variety of problems, including the absence of a clear measure, or marker, of stress. Many studies suffer from a major methodological flaw-the stress is not assessed until after the event. This introduces the potential for recall bias, which could increase the report- ing of stress among mothers with poor pregnancy outcomes. Also, concepts and definitions of stress vary substantially among studies, and sample sizes are often too small to isolate specific outcomes such as low birthweight. Finally, most current projects evaluating the effects of stress fail to control for smoking, a major correlate of low birthweight. Uterine Irritability The concept of uterine irritability and the possibility that certain external factors can stimulate or "trigger" uterine contractions are just beginning to be explored. Some studies suggest that excessive uterine activity may be a component of the events leading to preterm labor. For example, in a recent study comparing the rhythm of uterine contractions in the latter part of pregnancy over a 24-hour period in normal pregnancies and in pregnant women at high risk of preterm labor, the women with normal pregnancies were found to have long periods during which the uterus was quiet each night. In contrast, the high-risk women had uterine activity throughout the 24-hour cycle. Assessing the extent of uterine activity could be part of the surveillance of pregnant women at risk of preterm labor. Similarly, it may prove prudent to advise high-risk women to avoid those activities (which vary among individual women) that stimulate uterine contractions. Cervical Changes Recently, interest has grown in increasing the number of cervical assessments made in later pregnancy in order to identify changes that occur several days and sometimes even weeks before the onset of preterm labor. The presence of a short cervix or dilatation may lead the clinician to initiate some form of intervention to forestall preterm labor. Some prenatal care specialists advocate regular pelvic exams in later pregnancy to

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14 detect these changes, but others have expressed concern that these exams might lead to premature rupture of the membranes or other problems. No conclusive data exist on the risks of frequent cervical exams prior to term. Risk Assessment In an effort to use risk factor data to help structure prenatal care for pregnant women, researchers have developed a variety of techniques to measure risk status, including scoring systems (risk assessment instru- ments) to distinguish women at high risk of preterm labor and/or TUGR from women at Tow risk. The committee examined the predictive capabilities of 13 risk classification systems; a complete description of the results appears in the main report. An important finding was that the majority of these systems correctly identify as high risk about 65 percent or more of those pregnancies with eventual adverse outcomes. In addition to distinguishing high-risk from Tow-risk women, well- constructect risk assessment systems have the potential to reduce the misdi- agnosis both of {UGR anc! preterm labor and are helpful additions to clinical judgement in evaluating the risk of low birthweight. They also offer the possibility of grouping risk factors by their preventability or modifiability, thereby suggesting possible interventions. And risk asessment systems can encourage more appropriate referrals for care and more reasonable resource allocations for the management of preterm birth. The limitations of these instruments also must be recognized. First, be- cause the performance of a risk assessment instrument is to some extent dependent on the prevalence in a population of the adverse outcome being assessed, it is unlikely to produce the same results in every setting. Second, the instrument is a statement of probability only and cannot be viewed as a definitive predictor for a specific woman. Third, the instrument cannot be used in a rote manner to substitute for high-quality professional care. The not infrequent occurrence of Tow birthweight deliveries in Tow-risk women sug- gests that additional research is needed to improve the predictive capability of these systems. It also indicates that clinicians must be alert to the possibil- ity of Tow birthweight even in pregnant women judged to be at low risk of such an outcome. Research A major theme that emerges from the voluminous information on the causes and risks of low birthweight is the critical need for additional research in many areas. In particular, our understanding of the physiological pro- cesses involved in premature labor and JUGR is seriously inadequate. Efforts to prevent Tow birthweight will remain limited until more is known about basic causal mechanisms. In addition, more research is needed on specific risk factors, not only those somewhat speculative in nature, but also those clearly linkecl to low birth- weight. For example, little is known about the ways in which race exerts an effect on birthweight, and prevention strategies aimed at certain other risk factors, such as alcohol abuse, could be improved if there were a better definition of the magnitude of risk at various levels of consumption. Studies