Pregnancy changes not only the way a woman's tummy looks, but the community of bacteria living within her gut, according to a new study.

During the first trimester of the study subjects' pregnancies, researchers found little variation in the makeup of gut bacterial communities from woman to woman, and between pregnant and non-pregnant women.

During the third trimester, however, gut bacteria varied greatly from woman to woman, and there was less diversity of gut bacteria within any given woman, the study found.

“The findings suggest that our bodies have co-evolved with the microbiota and may actually be using them as a tool to help alter the mother's metabolism, to support the growth of the fetus,” said study researcher Ruth Ley of Cornell University.

Ley and colleagues obtained stool samples from 91 pregnant women during each trimester of pregnancy and analyzed the bacteria present.

They found that bacteria typically linked with good health decreased over the course of pregnancy, while bacteria associated with diseases generally increased. In addition, signs of inflammation in the gut increased.

These changes in gut bacteria may play a role in changing a pregnant woman's metabolism, the researchers said. Two changes that happen during pregnancy are an increase in the amount of body fat, and reduced sensitivity to insulin, the hormone that controls blood sugar.

In obese people, these changes are hallmarks of an unhealthy metabolism. But during pregnancy, the researchers said, the changes are healthy: They help the fetus to grow and prepare the mother's body for the energy that will be needed during breast-feeding.

Exactly what brings on the changes isn't clear, but the findings suggest that what a woman eats doesn't matter. “The changes in gut microbes were not related to diet, so we think the immune system or hormones play a role,” Ley said.

The researchers also transferred gut bacteria samples from women in each trimester to healthy, germ-free mice. They found that mice treated with the bacteria from the third trimester gained weight and had higher levels of inflammationand poorer sugar metabolism than mice treated with bacteria from the first trimester.

The basis of the interactions between a person's metabolism and her gut microbes — which can alter blood sugar regulation, and may underlie much of the current obesity epidemic — may lie in reproductive biology, the researchers concluded.

The study will be published Friday (Aug. 3) in the journal Cell.

Pass it on: The community of gut bacteria in a woman changes while she's pregnant.

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