African-American women are two to three times as likely to give birth prematurely as women of European origin. The reasons have generally been assumed to be socioeconomic. But scientists have now identified a possible genetic contributor: a variant relatively common among African Americans that affects the strength and resilience of the amniotic sac.
When a woman's "water breaks," the amniotic sac that keeps the fetus bathed in fluid ruptures. If this happens much before the usual gestation period of 39 weeks, she goes into premature labor. Called Preterm Premature Rupture of Membranes (PPRM), the condition affects three percent of pregnancies and accounts for one-third of premature births. Black women are at twice the risk of Caucasian women.
But why a woman's water breaks early isn't clear. Scientists led by physician Jerome Strauss of Virginia Commonwealth University in Richmond suspected that the fetus's genes might solve part of the puzzle. The team began considering the role of a protein called heat-shock protein 47 (HSP47), which helps boost production of collagen and is lacking in PPRM fetal membranes. Other studies have found two different versions the gene that makes HSP47. One, present in 12% of African Americans compared with just 4% of Caucasians, shows reduced activity in amniotic cultures.
To see if carriers of this version of the gene are more prone to PPRM, the team collected genetic data on infants delivered by 602 black mothers in four U.S. cities. Of these, 244 births involved premature sac rupture; the rest were normal term births. The mothers who experienced normal births were matched with the other mothers for age and reproductive history.
The scientists found a "significant" difference between the women whose water broke early and the women whose babies were born at term: Among the women with PPRM, 11.5 percent of fetuses had the key genetic variant, compared with 4.5 percent in the non-PPRM group. (The amniotic sac has the same genetic makeup as the fetus.) And, the researchers report online this week in Proceedings of the National Academy of Sciences, PPRM was more likely to occur if a fetus had two copies of this variant--one from each parent--rather than one.
The study is "potentially important," says physician Richard Cooper of Loyola University in Maywood, Illinois, a skeptic of genetic interpretations of racial differences in medicine. But he contends that the black-white gap in premature births has been narrowed by better prenatal care in recent years, so the mutation can probably only explain a "small proportion" of the racial difference.