Short pregnancy intervals may explain blacks' rate of poor birth outcomes

Differences between black women and white women in the spacing of pregnancies may account for some of the racial differential in pregnancy outcomes that increase the risk of infant mortality, according to the results of a recent study.(1) Black women who became pregnant within nine months of their previous birth had an elevated risk of delivering a premature, underweight infant, and interpregnancy intervals of this duration were fairly common among them. White women, however, were at elevated risk only if their pregnancies were less than three months apart, and relatively few whites had such closely spaced pregnancies.

To examine the racial differences in interpregnancy intervals and related outcomes, the researchers identified 1,922 women (624 whites and 298 blacks) who had two consecutive singleton pregnancies with the same partner between July 1983 and June 1993. All of the women were in the military or were dependents of military personnel and thus had access to free, high-quality health care; they delivered at an army hospital in Washington State.

Because the study was concerned mainly with pregnancy outcomes that may lead to infant deaths, the researchers focused on the premature delivery of low-birth weight infants in the second pregnancy. Infants delivered before 37 completed weeks of gestation were categorized as premature, and those weighing less than 2500 g were considered low-birth-weight, The investigators defined the interpregnancy interval as the period between the first delivery and the next conception (based on the second infant's gestational age at birth, as measured by standard obstetric dating criteria).

At the time of the second birth, the two racial groups were similar with respect to age and parity; on average, blacks had a slightly lower family income than whites. Women whose pregnancies were less than six months apart (30% of blacks and 18% of whites) were younger and had lower family incomes than those with longer interpregnancy intervals.

Black women were more likely than whites to be unmarried (9% vs. 3), and whites were more likely than blacks to be smokers (20% vs. 14%). The two groups of women were similar with regard to when they initiated prenatal care and whether they developed pregnancy-induced hypertension.

Adverse outcomes of the first pregnancy were more common among blacks than among whites; for example, 14% of black women had a low-birth-weight infant or premature delivery, compared with 9% of the white women. In addition, the prevalence of intrauterine growth retardation (birth weight for gestational age below the 10th percentile for all infants born at the medical center during the study period) was higher among the infants born to black women than among those born to whites--16% vs. 6%.

Black women also were more likely than whites to have an adverse outcome in their second pregnancy. Some 8% of black women and 3% of whites had a premature, low-birth-weight infant (p

To determine more precisely the interpregnancy interval at which the likelihood of bearing a premature, low-birth-weight infant changed significantly, the investigators reexamined the data using three-month intervals. They found that among black women, the prevalence of this outcome was higher if the period between pregnancies was less than nine months than if it was longer (12% vs. 4%); furthermore, a substantial proportion of black women (46%) had an interpregnancy interval of less than nine months. Among whites, a similar, but weaker, relationship emerged: Those whose pregnancies occurred within three months of the previous birth had a significantly increased risk of poor outcomes (12% vs. 3%), but relatively few white women (4%) had such closely spaced pregnancies.

Women whose first infant born during the study period was premature or low-birth-weight were 10.5 times as likely as those who had had a normal birth to subsequently bear a premature, underweight infant (p

Meanwhile, among high-risk women, the proportion of pregnancies with adverse outcomes differed significantly by interpregnancy interval for whites (46% at less than three months vs. 18% at longer intervals); for blacks, the number of infants was too small to show any clear association.

Using a stepwise logistic regression analysis to examine the impact of background factors on the likelihood of a poor outcome, the researchers found that white women were most likely to bear a premature, underweight infant if their preceding pregnancy had had an adverse outcome, if their interpregnancy interval was shorter than three months or if they smoked. When variables related to obstetric history were excluded from the analysis, prenatal care and an interpregnancy interval of less than three months were the factors most strongly associated with adverse outcomes.

Similarly, black women were at greatest risk of having an adverse outcome if they had previously borne a premature or low-birth-weight infant; interpregnancy intervals of less than six months and less than nine months were the next most important factors. These intervals were also the leading risk factors when obstetric history was excluded.

The finding that close spacing of pregnancies raises the risk of adverse outcomes, and that interpregnancy intervals frequently are short, especially among black women, has important implications for U. S. health policy, the researchers conclude. In contrast to many risk factors for adverse pregnancy outcomes, they observe, the spacing of pregnancies is potentially within individuals' control. Therefore, they suggest, public health strategies "should include counseling that strongly encourages intervals of nine or more months between pregnancies."

The author of an editorial addressing the study(2) points out that its reliance on a single outcome makes the results difficult to interpret, but notes the value of the focus on biological risk factors, since socioeconomic characteristics alone cannot explain racial differences in pregnancy outcomes. Further research, she comments, might explore the biological mechanisms that limit fetal growth or lead to premature delivery, as well as examine the characteristics of black women that put them particularly at risk of having adverse pregnancy outcomes.

In a subsequent issue of the journal in which the study was reported, the authors of a letter to the editor raise the issue of how the employment status of the women in the sample may have affected the results.(3) Pointing out that the physical exertion associated with active duty in the military is a documented risk factor in premature delivery, they question whether a racial difference in the proportion of women on active duty might account for some of the elevated risk of premature delivery observed among black women. In response, the investigators present data showing that although a higher proportion of black women than of white women in the study were on active duty, this factor did not explain the race-specific differences in pregnancy outcomes by interpregnancy interval.

References

1. J. S. Rawlings, V.B. Rawlings and J.A. Read, "Prevalence of Low Birth Weight and Preterm Delivery in Relation to the Interval Between Pregnancies Among White and Black Women," New England Journal of Medicine, 332:69-74, 1995.

2. E. Lieberman, "Low Birth Weight--Not a Black-and-White Issue," editorial, New England Journal of Medicine, 332:117-118 1995.

3. G. S. Berkowitz and M. Hatch, letter to the editor, New England Journal of Medicine, 333:386, 1995.

Copyright The Alan Guttmacher Institute Sep 1995
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