Twin Pregnancies: Not Enough Weight Gain Boosts Risk of Preterm Birth

A pregnant woman
The U.S. pregnancy rate is falling, according to a new report. (Image credit: Pregnancy photo via Shutterstock)

CHICAGO — For women pregnant with twins, not gaining enough weight in the second trimester may increase the risk of preterm birth, a new study suggests.

In the study, women carrying twins who gained less than 8 lbs. (3.6 kilograms) between weeks 20 and 28 of pregnancy were nearly three times more likely to give birth early (i.e., before 32 weeks of pregnancy), compared with women who gained more than 8 lbs. during those weeks.

The reason for the correlation is not known. However, the second trimester (which begins around week 12 to 14, and ends at week 28) is when pregnant women tend to put on weight most rapidly, as they gain fat and experience an increase in blood volume, said study researcher Dr. Kate Pettit, a maternal and fetal medicine specialist at the University of California, San Diego.

If women don't gain enough weight in the second trimester, then their overall weight gain in pregnancy is probably not going to be adequate, said Pettit, who presented the study here this week at the meeting of the American Congress of Obstetricians and Gynecologists. [Seeing Double: 8 Fascinating Facts About Twins]

The new finding confirms previous research linking poor overall weight gain in pregnancy to preterm birth, and "now highlights the likely crucial period of time of 20 to 28 weeks of gestation that may be a target for future interventional trials," Pettit said.

Women pregnant with twins are already at a higher risk for preterm birth than women carrying one child (called a singleton pregnancy): About 60 percent of twins are born prematurely, while the overall U.S. preterm birth rate is 11 to 12 percent.

There are currently no ways to reduce the risk of preterm birth in twin pregnancies. Although treatment with the hormone progesterone can lower the risk of preterm birth in singleton pregnancies, the same has not been shown in twin pregnancies, Pettit said.

If future studies confirm the new findings, then advising women about weight gain in the second trimester would be a simple intervention that could lower this risk, Pettit said.

Previous studies have found that only about half of pregnant women say they spoke with their obstetrician about weight gain in pregnancy. "There's definitely room for improvement in terms of the amount of counseling and monitoring and emphasis on weight gain during pregnancy," Pettit said.

The new study involved 489 women in San Diego who delivered twins between 2001 and 2013. Among women who did not gain enough weight between weeks 20 and 28 weeks of pregnancy, 37.6 percent gave birth before 32 weeks, compared with 15.2 percent of women who gained enough weight in weeks 20 to 28.

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Rachael Rettner

Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.