Low birth weight (LBW) infants weigh less than 2,500 g at birth and in the United States constitute 8% of all live births, the overwhelming majority born prematurely.1 Low birth weight may be responsible for as much as 10% of health care costs for children2 and has been linked to obesity,3 hypertension,4,5 diabetes mellitus,4 and reduced fertility6 in adulthood.
Low birth weight rates are influenced by maternal age, marital status, socioeconomic status, interpregnancy interval, race, and medical problems such as hypertension, prematurity, and health-compromising behaviors in pregnancy.7–16 Health behaviors routinely screened for, such as smoking, alcohol consumption, and illicit drug use, also contribute to the risk of premature birth. With the exception of maternal age and race, LBW-associated factors are potentially modifiable with proper education and provision of adequate health care for pregnant women. In this context, the relationship between health-compromising behaviors and LBW has been extensively studied.
Of all 3 behaviors, cigarette smoking17 has been most studied and strongly implicated in the causation of LBW. However, alcohol consumption13,18,19 and, less consistently, illicit drug use20–23 have also been implicated as playing important roles. Low birth weight rates have been shown to increase as the number of health-compromising behaviors and other environmental risk factors increase.13–15,20 Specifically, the combination of smoking and alcohol consumption significantly increases LBW rates over those of either behavior among married white women (over the age of 30 years) who give birth to singletons.13 Similar findings for combined smoking and alcohol consumption effects have been described among low socioeconomic black women.14
Smoking, alcohol consumption, and illicit drug use are risk factors for premature birth.14,23,24 Indeed, some studies show that neonatal costs of infants of cocaine-abusing mothers may be as much as 10 times greater than those of non–substance-abusing mothers and that those costs are mainly due to premature births.25 The effects of various combinations of 3 specific health-compromising behaviors (smoking, alcohol consumption, and illicit drug use) on LBW rates for the subgroup of pregnancies that reach term gestation has not been previously studied. (MEDLINE search from 1966 through 2003 using the terms “low birth weight,” “attitude to health,” “alcohol drinking,” “drugs of abuse,” “illicit drugs,” “recreational drugs,” “tobacco use disorder,” “pregnancy,” and “pregnant women”). To investigate these effects, we tested the hypothesis that, among infants born at term (≥ 37 weeks) gestation, LBW (term-LBW) rates increase with increasing numbers of health-compromising behaviors during pregnancy.
MATERIALS AND METHODS
Data were obtained on all infants who were born in Kansas City, Missouri, at term gestation (37 or more completed weeks of pregnancy) and on their mothers over a 13-year period, 1990 through 2002. The source of the data were the computer files at the Kansas City Health Department of birth certificates containing information on maternal and infant demographics.26,27 Institutional review board approval for analyses and publication of this study was deemed not necessary at the Kansas City Health Department.
For the purpose of this study we defined health-compromising behavior as maternal behavior that has been previously shown to be associated with compromised growth of the fetus; this definition was limited to cigarette smoking, alcohol consumption, and illicit drug use.8,13–16,19,20,28,29Term low birth weight (term-LBW) was defined as a birth weight of less than 2,500 g by 37 completed weeks of gestation.
Complete information was abstracted on variables of interest, namely, mothers' age, educational attainment (< 9 years, 9–12 years, or > 12 years), marital status (single or married), race (black or white), adequacy of prenatal care (using a modification of the definition used by the California Department of Health Services: adequate [first trimester, ≥ 9 visits], inadequate [third trimester, none], or marginal [outside of the criteria for the adequate and inadequate]),30 interconception interval (none [first complete pregnancy], < 18 months, or ≥ 18 months), Medicaid (yes or no), maternal medical risk factors for LBW (hypertension, renal disease, or cardiac disease), and self-reported smoking, alcohol consumption, and/or drug use during pregnancy, infants' birth weights, and gestational age.7,10,16 Inclusion criteria included term gestation, to eliminate LBW due to premature birth,21,23,31 and white and black infants,7,9 because of relatively smaller numbers of other racial/ethnic groups. Multiple births were also excluded because of the previously described association of multiple birth and LBW.32
Analyses were conducted using SPSS 13.0 (SPSS Inc, Chicago, IL). All variables of interest were categorical and were expressed as percentages and compared by groups using χ2 test. Correlation coefficients (Spearman) were calculated to determine the degree to which the variables were related to each other. Additional analyses were performed by multivariable logistic regression, with term-LBW as the outcome variable and the other variables (of interest) as independent variables. All variables were entered into the model because they have been previously shown to significantly influence LBW. Statistical significance for all analyses was P < .05.
The cohort comprised 78,397 mother-newborn pairs, which were 61% white, 16% less than 20 years of age, and 45% unmarried pregnant women. The distribution of other demographic characteristics by health-compromising behaviors is shown in Table 1. Briefly, blacks, single, women over 20 years old, and those with 9–12 years of education, on Medicaid, and inadequate prenatal care were more likely to report engaging in multiple health-compromising behaviors. Smokers were more likely than nonsmokers to use alcohol or illicit drugs during pregnancy (13.0%, 95% CI 12.5–13.5% and 2.2%, 95% CI 2.1–2.3%, respectively, P < .001). There was a trend toward heavier smoking and a significant increase in heavier drinking (> 5 drinks/week) with increasing numbers of health-compromising behaviors (Tables 2 and 3).
The overall term-LBW rate was 3.3% and was higher among pregnancies that reported one or more health-compromising behaviors (5.6%, 95% CI 6.0–6.8%) than those that reported none (2.6%, 95% CI 2.5–2.8%). Term-LBW rates increased with increasing numbers of behaviors: 2.6% (none), 5.5% (1 behavior), 10.8% (2 behaviors), and 18.5% (3 behaviors), P < .001 (Table 4). However, term-LBW rates were significantly increased mainly for behaviors that included smoking. The term-LBW rates were also higher for women with the following characteristics: < 20 years old, black, single, on Medicaid, first live birth or short interpregnancy interval, 9–12 years of education, medical risk for LBW, and marginal or inadequate prenatal care (Table 5)
The unadjusted odds of term-LBW, with the exception of alcohol consumption alone, increased exponentially with increasing numbers of health-compromising behaviors, irrespective of their combinations (Table 6). All covariates were statistically significantly correlated with each other (P < .001). However, the following covariates had correlation coefficients greater than .200: race with marital status (r = .480, P < .001), Medicaid (r = .358, P < .001), and prenatal care (r = .217, P < .001); prenatal care with education (r = .231, P < .001), Medicaid (r = .238, P < .001), and marital status (r = .266, P < .001); education with Medicaid (r = .445, P < .001) and marital status (r = .394, P < .001); Medicaid with marital status (r = .546, P < .001); and medical risk for LBW with health-compromising behaviors (r = .220, P < .001).
All variables were entered in the model because of previously demonstrated relationships with LBW, in spite of their correlation with each other. Multivariable logistic regression adjusting for all covariates showed that the odds of term-LBW increased only for behaviors that included cigarette smoking during pregnancy (Table 6). Separate analyses of blacks and whites did not reveal differences in the prediction of risks of term-LBW from health-compromising behaviors.
This study shows that, among infants born at term gestation, LBW rates increase with increasing numbers of a woman's health-compromising behaviors during pregnancy. Cigarette smoking is the major predictor of term-LBW, and its growth-restricting effect may be augmented by alcohol consumption.
The term-LBW rate in this cohort, 3.3%, is much lower than the overall LBW rate of 7.6% reported in the United States because the majority of LBW infants are born prematurely.1 Of note, the term-LBW rate in this cohort increases 6-fold when pregnancies that reported all 3 behaviors are compared with those that report none. This finding supports that of others with respect to various combinations of 2 of these 3 health-compromising behaviors13,14 and for the first time shows that there might be an additional effect on term-LBW rates from engaging in a third health-compromising behavior (MEDLINE search from 1966 through 2003 using the terms “low birth weight,” “attitude to health,” “alcohol drinking,” “drugs of abuse,” “illicit drugs,” “recreational drugs,” “tobacco use disorder,” “pregnancy,” and “pregnant women”).
Various growth-restraining mechanisms have been postulated to explain the development of LBW in fetuses exposed to these health-compromising behaviors.28,29,33 Impaired oxygen and nutrient delivery from mother to fetus because of smoking and a direct fetal-teratogenic effect of alcohol have been suggested as potential mechanisms for their fetal growth–limiting effect.28,33 However, the growth-limiting effect of illicit drug use has always been attributed, in some reports, to premature birth23 and, in other reports, to the associated smoking and/or alcohol consumption.29 By limiting the study to infants of term gestation, the role of premature birth in LBW was eliminated. Of note, pregnancies that reported alcohol and/or drug use did not appear to be at greater risk of term-LBW than pregnancies that reported abstinence. However, the addition of either behavior to smoking increases the risk of term-LBW 2- to 4-fold, an effect that is more marked when smoking is combined with alcohol. Furthermore, the risk from all 3 behaviors, although not significantly greater that that observed when smoking is combined with either alcohol or drug use, increases 3-fold over that of abstainers.
There are a number of plausible explanations for our findings. Firstly, there may be synergism in the growth-limiting effects of the various exposures, particularly for the combined exposures of smoking and alcohol use.23,28,33 Secondly, increase in the numbers of health-compromising behaviors is associated with a corresponding increase in the numbers of other socioeconomic/demographic risk factors for LBW. Support for the role of other factors is found in the observation of an absent term-LBW effect from drug (with/without alcohol) use and a reduced LBW effect from the combination of all 3 behaviors after correcting for demographic and medical risk factors. The interrelatedness of these demographic variables has been well described by McCalla et al.34 They showed that, although illicit drug users have a wide range of social problems that compromise fetal growth and development and are in greater need for prenatal care, they were less likely to make use of prenatal care services.34
Thirdly, there is evidence that the “dose” or quantity of substance exposure (tobacco or/and alcohol) increases with the number of health-compromising behaviors engaged in (Tables 2 and 3). In general, pregnant smokers were 7 times more likely than nonsmokers to use alcohol and/or drugs, and the rate of heavy smoking and moderate/heavy drinking increased with the number of health-compromising behaviors. This observation confirms the observation of others that significantly more pregnant smokers engaged in heavy alcohol consumption than pregnant nonsmokers13 and that pregnant heavy-smokers are more likely to engage in illicit drug use than light smokers or nonsmokers.35
Contrary to our expectations, the risk of term-LBW in pregnancies that reported alcohol and/or drug use is similar to that of abstainers, after adjusting for other risk factors. Some studies,15,31 including a prospective study,31 show that mild-to-moderate alcohol consumers have a lower risk of LBW than abstainers. Because of the “dose” heterogeneity of alcohol consumption in our study, we speculate that the previously described “growth-protective” effect of mild-to-moderate drinking may have been counter-balanced by the growth-inhibiting effect of heavy alcohol consumption.13–15,31,36–39 Also consistent with the observations of others at all gestational ages, the absence of a specific term-LBW effect from illicit drugs, with or without alcohol, may be due to overriding effects from other concomitant demographic and environmental risk factors.7–16,20–23
Of interest, but not directly related to our study objectives, we confirmed the association between term-LBW outcome and other previously described factors such as demographic factors (marital status, education, Medicaid, prenatal care, maternal age, race), medical risk factors (hypertension, kidney disease, or heart disease),7–16,40–42 and interpregnancy interval.12 Single mothers had a higher rate of LBW than married mothers, a finding that may support a role for social support in pregnancy outcomes41,42 and its secondary effects on other health behaviors during pregnancy, particularly the timely use of prenatal care. Supporting this notion is the observation by others that cohabiting mothers have an intermediate rate of LBW to single and married mothers.41 It is also conceivable that the effects of single motherhood42 and race43 may also be mediated through the stress associated with poorer social support and socioeconomic circumstances.
Our ability to generalize these findings may be limited for a number of reasons. Firstly, the birth certificate records do not provide information on the full range of maternal medical (complicated diabetes mellitus, renal diseases, and hematological disorders) and fetal (chromosomes, syndromes) problems that have the potential to cause term-LBW but that also may be associated with engaging in health-compromising behaviors. Secondly, we were unable to measure other potential confounders, predisposing factors, mediators, and effect modifiers such as maternal nutrition and stress. Although there are no quantitative differences in the diet of pregnant women who engage in multiple health-compromising behaviors, qualitative differences, specifically relative vitamin deficiency, may impact fetal growth.44–48 Others have shown that stress may contribute to LBW42 and initiate health-compromising behaviors.49 Thirdly, due to the legal consequences of admitting to some of these behaviors during pregnancy, it is conceivable that there was underreporting (and classification error), especially of illicit drug use.50 Despite the perennial concern about underreporting, although the intensity of a behavior may be difficult to validate, the actual report of the behaviors in surveys and birth certificates has been shown to be reasonably reliable.26,27 Nonetheless, the latter 2 limitations, especially underreporting of health-compromising behaviors, should have weakened the strength of observed associations. Therefore, the robustness of our findings, despite excluding premature births and correcting for the usual covariates,24,34 and the biological plausibility for these agents to work synergistically23,28,33 provide good evidence for probable cause-effect relationship between the observed term-LBW rates and multiple combinations of health-compromising behaviors.
In conclusion, term-LBW rates increase significantly with numbers of health-compromising behaviors in pregnancy. Smokers are more likely than nonsmokers to engage in multiple health-compromising behaviors and be at greatest risk of term-LBW, especially when smoking is combined with alcohol consumption. These observations provide additional evidence of the need to heighten the sensitivity of the health care provider and the pregnant woman to the dangers of smoking, in particular, and the incremental dangers of multiple health-compromising behaviors, in general.
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