Preventing Low Birthweight (1985) / Chapter Skim
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2. Etiology and Risk Factors
2. Etiology and Risk Factors
Pages 46-93
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From page 46... ...
presence in an individual woman indicates Etiology As described in the previous chapter, the term low birthweight can refer to three often intertwined outcomes of pregnancy: preterm delivery, intrauterine fetal growth retardation (TUGR) , and a combination of both.
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From page 47... ...
Knowledge about the etiology, initiation, and maintenance of preterm labor is limited. 2 9 A The underlying pathophysiology is postulated on the basis of variations from the normal patterns of hormonal effects detected in animals and man or on the basis of
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From page 48... ...
Several of these conditions are discussed later in the chapter as risk factors for preterm birth, although it is apparent that the distinction between a r isk factor and a causal mechanism is not always clear. Finally, in many cases of premature birth, no association with a pathologic factor can be identified.
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From page 49... ...
For example, many studies analyze the relationship of a given risk factor to slow birthweight" rather than to the more specific outcomes of preterm delivery and intrauterine growth retardation; also, these studies commonly rely on estimates of gestational age, which often are
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Birth certificates in most states do not provide information on such factors as maternal height, weight, weight gain during pregnancy, or smok ing practices . A chart listing the principal risk factors for low birthweight is shown in Appendix A
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Inadequate plasma volume expansion G Progesterone deficiency This grouping leads to the observation that many of the risk factors for low birthweight (categories I, II, and IV of the table)
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From page 52... ...
These include the demographic risk factors of race, age, and socioeconomic status; the medical and obstetr ic r Asks of hypertension/preeclampsia, diabetes, obstetric history (including previous induced abortion) , multiple pregnancy, and infection; nutr ition; the behav ~ oral and environmental r isks of smok ing and alcohol use; and iatrogenic prematur ity.
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From page 53... ...
When matched for both age and education, blacks are still at higher risk of low birthweight.22 See also Appendix Table B.9. Black women are more likely than white women to delay initiation of prenatal care.2 ~ However, when receipt of prenatal care is held constant, black women are still at increased risk of delivering a low birthweight baby.2 3 A limitation of most studies in this area is the
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From page 54... ...
At almost every combination of height and weight, and for almost every combination of weight and weight gain, black women were at hither risk of low birthweight Than white women. Furthermore, black women were at higher risk of low birthweight at all combinations of age and parity.20 Finally, it has been shown that women whose last infant was of low birthweight are at increased risk of low birthweight in the current pregnancy.24 Yet in the Collaborative Study, when women were stratified by whether or not their last child was low birthweight, blacks were still at increased risk of low birthweight during the
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From page 55... ...
recently showed, for example, that the Spanish surname infant mortality rate may appear lower than it actually is because of uneven reporting of infant mortality in this population and that it is therefore an inaccurate indicator of Mexican-American health status.2s Similar data problems may skew reporting of low birthweight. The large difference between black and Mexican-American low b~rthweight rates also raises the possibili ty that cultural differences may play a role in pregnancy outcome.
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From page 56... ...
Teenage mothers, particularly the youngest (under age 15) , have many other risk factors that could be responsible for an adverse pregnancy outcome.
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From page 57... ...
is clearly associated with an increased risk of low birthweight and preterm delivery. The literature suggests that at least some of the excess risk is due to other var tables that are also associated with both low social class and low birthweight.
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From page 58... ...
Another aspect of low socioeconomic status that may help to explain its relationship to low birthweight is derived from two separate bodies of data: the information that various classes of genital tract infections are associated with low birthweight and the possibility that these organisms may be more prevalent in low SES women. The subject of genital tract infection is discussed In more detail later in this chapter .
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From page 59... ...
and a variety of maternal factors. They reported risk factors for IUGR as mother's age, number of previous pregnancies, birth of a previous low birthweight infant, smoking, and elevation in diastolic blood pressure.
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Premature delivery may be necessary for pregnant women with insulin-dependent diabetes complicated by diabetic vasculopathy. In such cases, preterm delivery may be indicated because of worsening maternal retinopathy, nephropathy, or hypertension.
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From page 61... ...
Based on a detailed study of the weights and gestational ages of all -births in Norway from 1967 through 1973, Bakketeig concluded that a premature first birth is the best predictor of a preterm second birth, and that growth retardation of a first birth is the most powerful predictor of growth retardation during a second pregnancy. The risk of the birth of a subsequent low birthweight infant is 2 to 5 times higher than average for mothers who have had a previous low birthweight delivery and increases with the number of prior low-weight births.24 Previous fetal and neonatal deaths also are strongly associated with preterm low birthweight; again, the risk increases as the number of previous poor fetal outcomes goes up.
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From page 62... ...
is that research is needed to investigate further the relationship of induced abortion to subsequent pregnancy outcome. The fact that some studies have suggested an increased risk of preterm delivery associated with induced abortion in some c i rcumstances , coupled w, th the large number of such procedures in the United States annually, under score the importance of this research topic.
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From page 63... ...
Genitourinary Infection of the Mother Infections of the genitour inary tract also are relevant to low birthweight . Bacter fur ia is present in 3 to 8 percent of pregnant women and varies with parity; the incidence is higher in women of parity greater than three.54 Untreated or inadequately treated symptomatic urinary tract infections, which may include pyelonephritis, are known to have adverse effects on both mother and fetus, including low birthweight.
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the association between certain pathogens and low birthweight also centers on their possible role in triggering preterm labor. Bejar et al.
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From page 65... ...
Human famines caused by war are an unfortunate analogue of the animal laboratory nutrition reduction studies and have been examined to shed light on the impact of dietary restrictions on human birth outcomes. Studies of the effects of famine on pregnancy outcome during World War II in Leningrad70 and Holland indicate that extreme, sustained nutritional deprivation can compromise pregnancy outcome and increase the incidence of low birthweight.
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From page 66... ...
Data from the Collaborative Perinatal Study clearly document that increased maternal weight gain is related to larger b~rthweight, decreased low birthweight rates, and decreased neonatal mortality.20 76 Low birthweight occurred 4 times more frequently among women who gained less than 14 pounds than among those who gained 30 to 35 pounds. Consistent with such data are those of Taffel and Keppel, who recently explored the relationship between a mother 's weight gain during pregnancy and the occurrence of low birthweight by analyzing data from the 1980 National Fatality and Fetal Mortality Surveys.
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From page 67... ...
The relationship and trade-offs between these two maternal nutrition systems remain to be fully explored. In sum, the magnitude of nutritional effects on low birthweight is not easily assessed because nutritional status is cliff icult to isolate from other socioeconomic characteristics and because of the complicated relationship between prepregnant weight and weight gain dur ing pregnancy.
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From page 68... ...
Although exact rates of smoking dur ing pregnancy are not known, a 1980 Surgeon General's report estimated that 20 to 30 percent of pregnant women in the United States smoke. ~ 9 Such estimates are consistent with f igures on smoking rates for U
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From page 69... ...
reported on a prospective study of more than 30,000 pregnancies in women who were part of a health maintenance organization; the study demonstrated a significant increase in the risk of IUGR associated with drinking one to two drinks per day even after adjusting for other important risk factors, such as maternal age, race, education, marital status, maternal weight for height, smoking, parity, prior reproductive history, hypertension, and preeclamps~a.9 2 An unpublished report by Graves et al., based on an analysis of the data from the 1980 National Natality and Fetal Mortality Surveys, also demonstrated a significant increase in the rate of low birthweight in married women drinking one or more drinks per day during pregnancy. Again, the researchers controlled for many other variables known to be associated with the rate of low birthweight, including maternal age and race, family income, maternal prepregnant weight and weight gain, maternal and paternal weight-to-height ratios, and maternal smoking.93 Because moderate alcohol use during pregnancy may pose added risks to fetal development, common sense suggests avoiding both heavy and moderate alcohol use in pregnancy.
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From page 70... ...
Quilligan estimates that the total number of such operations could be reduced by more than ha~f.~°° Evolving Concepts of Risk This section explores several risk factors more speculative in nature than those described earlier. They are currently being studied in a variety of settings and may over time evolve into important predictors of low birthweight.
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From page 71... ...
Low prepregnancy weight, poor weight gain during pregnancy, and hypertension were also associated with lower birthweights.~ 02 A similar association between prematurity and some occupational categories was noted by Mamelle et al.~03 In contrast, the Card~ff Birth Survey found virtually no ill-effects associated with employment. In fact, a significantly higher proportion of births were growth retarded in the unemployed groups than in the employed groups, even after controlling for adverse medical and obstetric history.~°~ In general, the work-related studies and a major study of overall physical stress by Papiernik et al.~04 indicate that there is probably some association between low birthweight, manifested as both IUGR and prematurity, and activities that require long periods of standing or other substantial amounts of physical stress.
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From page 72... ...
In some studies, the proportion of women with evidence of adverse psychological states has been found to be higher among women who had deliver ies considered "abnormal" by their obstetricians ° 6 - ~ ~ 3; this effect appeared to depend partly on Me timing of the stress in relation to delivery ~ 2 and on the presence of psychosocial supports. ~ ~ 3 In other studies, however, no association has been found between increased stress and adverse pregnancy outcome; for example, no differences in outcome were noted between women refused abortions and comparison groups.~4~~6 Many studies have noted a relationship between stress and conditions that increase the r isk of IUGR and preterm labor, such as toxemia, but data supporting a specific relationship between maternal psychological distress and low bir thweight are not strong.
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From page 73... ...
~ 2 4 Several studies have suggested that cervical assessment may be another means to detect heightened risk for preterm labor. For example, part of Papiernik's and Creasy's approach to identifying women at risk of preterm delivery, described in Appendix C, involves serial pelvic exams to evaluate the cervix -- pr imar fly its shortness and state of dilatation.
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From page 74... ...
Prospective randomized trials are needed to determine whether plasma volume expansion prevents preterm labor in women with low plasma volumes. Progesterone Deficiency Progesterone, a key hormone for the maintenance of pregnancy, preserves uterine quiescence.
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There are limited data to suggest that low progesterone levels precede preterm labor.~32 134 Studies to show the efficacy of progestational agents In the prevention of preterm labor are limited.
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but this always involves a trade-off. Selecting a lower score (i.e., fewer risk factors)
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From page 77... ...
Predictive Value - (of low risk) = a/albC = d/c+dC "Preterm labor or intrauter ine growth.
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From page 79... ...
79 ~ ~ ~ cn ~ ~ 3 0 ~ :~ O ~ ~ O m s ~ m s ~ m ~4 ,.; 4~ _~ ,~ v ~ ~ s 1 o (U to 1 0 G1 to 1 V k ~ ~ ~~ ~ ~ ~ E~ u~ ~ ~ v ~ O vt~ O v— O V~ ~ V ~ ~ V~ 4~ s: ~ `: ~ ~ V ~ V .~: ~ ~ ~ s 0 ~ ~ ~ 0 ~ ~ ~ ~ ~ V ~ ~ ~ 3— ~ 0 :~ — ~ ~ :~ — ~.
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From page 81... ...
81 ~r a' E cs t8 C: vlm ~C 0 3 1 cq 1 5 ~: .
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From page 82... ...
Improving Risk Assessment Fur ther ref inement of our knowledge about the etiology, natural history, and epidemiology of low birthweight, and about associated risk factors, will contr ibute to improvements In the science of risk assessment. Information is needed to permit a clearer delineation of which outcomes to predict, which factors to monitor, and which weights to assign to individual factors.
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From page 83... ...
Stress, for example, may be a s ignif icant r isk factor for both IU=R and preterm labor, but more research is needed to understand the nature and magnitude of this risk. Other possibilities that merit study include, for example, the role of selected genitour inary infections in low birthweight, the natural history of uterine activity throughout pregnancy (to determine the value of uterine activity assessment as an index in evaluating the risk of preterm labor)
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From page 84... ...
15. Rliegman R and Rins K: Intrauterine growth retardation: Determinants of aberrant fetal growth.
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From page 85... ...
National Center for Health Statistics: Factors Associated with Low Birth Weight: United States, 1976. Prepared by S Taffel.
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From page 86... ...
36. Low JA and Galbra~th RS: Pregnancy characteristics of intrauterine growth retardation.
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From page 87... ...
47. Khouzami VA, Ginsburg OS, Da~koku NE, and Johnson JW: The glucose tolerance test as a means of identifying intrauterine growth retardation.
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From page 88... ...
67:501-527, 1954. Singer JE, Westphal M, and Niswander K: Relationship of weight gain dur ing pregnancy to bir thweight and ~ nfant growth and development in the first year of life: A report from the
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From page 89... ...
91. Ouellette EM, Rosett HL, Rosman NP, and Weiner L: Adverse ef feats on offspr ing of maternal alcohol abuse dur ing pregnancy.
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From page 90... ...
106. Jones AC: Life change and psychological distress as predictors of pregnancy outcome .
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From page 91... ...
Mhango C, Cates W Jr., Slovis B and Freeman M: Women refused second-trimester abortion: Correlates of pregnancy outcome.
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From page 92... ...
136 . Johnson JWC, Austin AL, Jones GS, Davis GH, and Ring TM: Eff icacy of 17 alpha hydroxyprogestrone caproate in the prevention of preterm labor .
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From page 93... ...
and McNellis D: Intrauter ine growth retardation: Identification of research needs and goals. Seminar Per~natol.
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