For the first time, doctors have identified a gene linked to an increased risk of premature birth among black women.

The discovery is likely to be controversial, as are many scientific studies drawing connections between genetics, race and disease.

The underlying issue is enormously significant: Rates of pre-term births among black women are two to three times higher than they are for white women.

For years, scientists have wondered whether genetics was implicated, in addition to environmental factors such as smoking or socioeconomic factors such as poverty.

Now, they have found evidence that this does, indeed, appear to be the case.

At issue is a variation in a gene that directs the production of collagen, a key component of the amniotic sac that lines a pregnant woman’s uterus. The sac holds the waters that cushion a baby inside the uterus.

Women with the variant gene are more prone to rupture the sac.

This “premature rupture of membranes” is responsible for about 40 percent of premature births in the United States, more than any other cause, said Dr. Roberto Romero, chief of the perinatology research branch at the National Institute of Child Health & Human Development.

The ruptures are more common among black women, as is the gene, according to the new study, reported online Monday by the Proceedings of the National Academy of Science.

“If you think of collagen as you would think of a rope, this (genetic variation) means that the fibers of the rope aren’t as tightly wound,” said Dr. Samuel Parry, a scientist at the University of Pennsylvania’s Center for Research on Reproduction & Women’s Health.

In turn, that means the amniotic sac may be more vulnerable to environmental stress.

“We’re not saying this gene causes prematurity. But it could be a contributor, along with other genetic and environmental factors,” said Dr. Jerome Strauss III, lead author of the study and dean of the medicine school at Virginia Commonwealth University.

Separately, Harvard University researchers reported Monday that a genetic-risk factor “doubles” the likelihood of prostate cancer in younger black men.

The purpose of the research, scientists say, is to better understand what causes disease and come up with better treatments.

Some day, Strauss speculated, it might be possible to give genetic tests to women before they become pregnant, identify those at risk of developing problems and target interventions that maximize a woman’s chance of having a healthy pregnancy.

Relatively simple interventions are possible, Strauss said. For instance, smoking increases the likelihood of an amniotic sac rupture, so women already at risk for this problem should stop smoking while pregnant, he said.

Vitamin C is important in “collagen synthesis,” Strauss said, so “you’d also want to make sure you’re getting enough Vitamin C in your diet.”

Dr. Nancy Green, medical director of the March of Dimes, called the new study exciting and said it was part of an “evolving awareness” that prematurity has genetic and environmental roots, like other complex illnesses such as asthma or heart disease.