Black Women Have Excess Risk of Preterm Babies

ST. LOUIS, Feb. 9 -- African-American women, compared with white women, have preterm babies more frequently and deliver prematurely at an earlier gestational age, found researchers here.

In a review of data on more than 700,000 births in Missouri from 1989 to 1997, Louis J. Muglia, M.D., Ph.D., of Washington University, and colleagues, found that black women were three times more likely than white women to deliver at some time from 20 through 34 weeks of gestation, and nearly four times more likely to deliver from 20 through 28 weeks.

And although all women with a history of pre-term delivery are at increased risk for subsequent preterm births, African Americans were five times more likely than whites to have repeat early deliveries, the investigators reported in the February issue of the American Journal of Obstetrics & Gynecology.

The authors also found that among women with recurrent premature delivery, 47% delivered within two weeks of the gestational age of their first preterm infant, suggesting that genetic factors were at play.

"In our population-based study, we found not only an overrepresentation of blacks in populations of preterm and recurrent preterm births but also that the gestational ages at which the preterm births occur are skewed to the earliest gestations when adverse medical outcomes are most likely to arise for the infant," the investigators wrote.

"We postulate that although preterm birth is a detrimental outcome in pregnancy, it may be a result of a selective advantage, conferring inflammatory protection against other disease processes," they continued. "This selective advantage phenomenon has been well described for diseases afflicting the black race, particularly sickle cell disease, glucose-6-phosphate dehydrogenase deficiency, and nitrous oxide synthase polymorphisms and their effects on the incidence of malaria."

The authors conducted a population-based cohort study to determine whether being black was an independent risk factor for extreme preterm birth and for the frequency of recurrent preterm births at similar gestational ages.

They drew on the Missouri Department of Health's maternally-linked database of all births occurring in the state from 1989 through 1997, looking for specific factors associated with recurrent preterm delivery.

They found that African-American women had a fourfold risk for recurrent preterm birth (adjusted odds ratio 4.11, 95% confidence interval 3.78 to 4.47).

Black women had a threefold higher rate of birth at 20 to 34 weeks of gestation than white women (relative risk, 2.99; 95% CI, 2.89 to 3.08), and a nearly fourfold higher risk for extreme preterm birth, defined as 20-28 weeks of gestation. Compared with white women, the relative risk that a black woman would give birth to an extremely premature infant was 3.71 (95% CI, 3.43 to 4.00).

"Other factors associated with a higher rate of preterm delivery were mothers who were teenagers, did not graduate high school, received no prenatal care, experienced complications of labor such as placental abruption and pregnancy induced hypertension, were underweight, or were recipients of Medicaid or food stamps, as compared with the total population," the authors noted.

Black women had preterm births at earlier gestational ages than white women (31 versus 33 weeks' median age), and multiparous black women also had higher multiplicity of prematurity.

The odds ratio that a black woman would have three preterm births compared with a white woman was 2.14 (95% CI 1.49 to 3.07) and the odds ratio for four preterm births was 5.09 (95% CI 1.26 to 20.51).

"Whereas the racial disparity in extreme preterm birth could be partially explained by disparate access to medical care or other environmental variables, our data suggest that the proposed genetic component of preterm birth may be a greater etiologic contributor than previously recognized, because racial differences in preterm birth severity and recurrence persist in this cohort, even after adjusting for known medical and socioeconomic confounders," the investigators concluded.

The authors pointed out that "a limitation of the database is the inability to link births with extended familial relationships. To establish genetic implications of preterm birth, it would have been useful to be able to link a mother who experiences preterm birth with other family members who have also experienced preterm birth. By limiting our definition to recurrent preterm birth in one mother, we acknowledge the confounding nongenetic risk factors that must be considered. We corrected for socioeconomic status and other factors to isolate genetic contributors."