Why Are So Many Black Births Preterm?

Medical researchers have known for at least a decade that black women are at greater risk than whites for delivering babies prematurely. Why the disparity? A study out today in the American Journal of Obstetrics & Gynecology strongly suggests that genetic factors play a role. Researchers at Washington University School of Medicine in St. Louis reviewed the birth statistics of all women who delivered babies in Missouri between 1989 and 1997. Even after adjusting for risk factors like lack of prenatal care, smoking, diabetes and hypertension, African-American women were twice as likely as Caucasian women to give birth between 20 and 35 weeks of gestation (babies are considered full term if they're born between 37 and 42 weeks). NEWSWEEK's Anna Kuchment spoke with the study's lead author, Louis Muglia, director of the school's Center for Preterm Birth Research.

NEWSWEEK: What do you believe causes such a high percentage [roughly 8.8 percent of those who gave birth between 20 and 35 weeks of gestation] of African-American women to deliver babies early?
Louis Muglia:
We don't know the pathways responsible for normal-term birth or those that are responsible for preterm birth. Knowing the biology is something we're hoping to do. There could be risk factors not captured in our study. We suggest there is a significant genetic component that we could learn a lot from by investigating further.

What is the significance of your finding that African-American women who delivered their first child early, are likely to give birth to later children at around the same point in their pregnancy?
To me, that suggests that an acute infection or some kind of environmental exposure is less likely. It seems as though something in the mom or baby that determines the timing of birth could be shifted a little bit by genetic variations. Until you have the genes in hand to really do the experiments, we won't know for sure. But it's a model supported by the data.

What do researchers know about what causes a woman to go into labor?
It's actually very interesting. There has been decades of work in animals, where there's a clear idea of what occurs in the hormone levels that signal that birth is about to occur. But then when you look at human pregnancies, those same signals don't seem to be in place. The most significant one is a fall in progesterone. In people, that doesn't seem to occur, which is why we need to do the human studies. There could be 500 different genes that would be likely candidates involved in the timing of labor. We do know what makes the uterus contract more strongly: oxytocin and prostaglandins. But we don't know what starts that process.

Is anyone working on isolating those genes?
We are, and lots of people are. There was a nice study published by Jerome Strauss's group [in October's Proceedings of the National Academy of Sciences] that showed that African-American women who have a preterm rupture of the membranes are more likely to have a genetic variant that's causative for that. So some genes associated with preterm birth vary as a function of race.

Do physicians currently follow African-American women more closely, based on these numbers?
The most likely risk factor for giving birth preterm is having had a previous child born preterm. So all those women are followed more closely.

Is there anything that pregnant women who are at greater risk of preterm delivery can do to help lower their chances of giving birth early?
Women who have had preterm deliveries should pay particular attention to nutrition—taking prenatal vitamins—and continuing with close prenatal care.

Your study alludes to a national epidemic of preterm birth. Can you talk more about that?
Nationally, 12.5 percent of all babies are born earlier than 37 weeks. That's a 20 percent increase over the last 20 years. And it's interesting that, if you look at other developed countries, the rate is much lower. In Scandinavian countries, it's only about 5 percent. It's unclear what causes this disparity, but differences in nutrition, prenatal care and poverty levels may contribute. Obviously, the increase in the United States isn't accounted for by a change in our genes. So it's probably a complex interaction between genes, environment and social factors.

What about other factors, like the increase in multiple births?
That has contributed, but I don't believe it corrects for the increase in frequency. I think it's multifaceted: I think it's prenatal care, genes and the environment, including nutrition and timing between pregnancies that all interact together to lead to this growing rate of preterm birth in the United States.