Moderate weight gain may be healthiest in twin pregnancies

Pregnant women may be “eating for two” - or more - but when it comes to twin pregnancies, gaining too much weight may be as bad as gaining too little, a U.S. study suggests.

There isn’t a lot of evidence for an ideal amount of weight gain in a twin pregnancy, the study team writes in Obstetrics & Gynecology. But in a large analysis of twin pregnancies, they found that both very high and very low weight gain was associated with more preterm births and infant death.

Gaining too little weight was also linked with having babies that were small for their gestational age, and gaining too much was tied to overly-large babies and cesarean deliveries.

“Twin pregnancies have high rates of complications, so it is important to identify factors that we can modify during pregnancy to lessen these risks,” said Lisa Bodnar of the University of Pittsburgh in Pennsylvania, who led the study.

During the past 40 years, the number of twins born in the U.S. has doubled, Bodnar noted, and twins account for about 3% of births but more than 20% of preterm births. Women carrying twins are also more likely to experience diabetes, preeclampsia and cesarean deliveries.

The National Academy of Medicine recommends that women who are normal-weight or underweight gain 17-25 kilograms (37-55 pounds) during a twin pregnancy, and that overweight women gain no more than 11-19 kg (24-42 lbs).

“We’ve known for a long time that twin pregnancies carry a much higher risk of poor outcomes, and weight gain is a very common conversation for a pregnant woman and her doctor to have,” Bodnar told Reuters Health by email. “However, the lack of evidence on what that optimal range is left women and their doctors to make educated guesses or not to discuss weight gain at all.” Bodnar and colleagues analyzed data from nearly 55,000 twin births in Pennsylvania from 2003 to 2013. Small-for-gestational-age and preterm birth before 32 weeks were most common among women who were underweight before pregnancy, they found. And large-for-gestational-age birth and cesarean delivery were more common with higher pre-pregnancy body mass index (BMI, a measure of weight relative to height).

For normal-weight women, pregnancy weight-gain also appeared to influence risk. For example, compared with a weight gain of 20 kg, or about 44 lbs, a gain of 27 kg, or 60 lbs, was associated with 2.2 fewer cases of small-for-gestational-age but 2.9 more cases of large-for-gestational-age and 3.7 more cases of cesarean delivery.

Over the study period, weight gains in nearly one in three women fell outside the National Academy of Medicine guidelines, the authors note.

“We are not saying that gaining within the weight ranges identified by our study is necessarily best for the health of the mother and her babies, but simply that gaining above or below them carries greater risk of poor health,” Bodnar said. “Ongoing conversation between a patient and her obstetrician about weight gain while pregnant is an important part of prenatal care.” Researchers are still trying to find the optimal range, especially as obesity increases nationwide, which underscores the importance of nutrition and gestational weight gain, said Dr. Roger Newman of the Medical University of South Carolina in Charleston, who wasn’t involved in the study.

“There is nothing that a mother can do to help ensure a happy outcome of her twin or triplet pregnancy that is more important than achieving an optimal nutritional balance and gestational weight gain,” said Newman, who co-authored the book “When You’re Expecting Twins, Triplets or Quads.” “Many women are afraid to gain the amount of weight advised and are resistant,” he said by email. “Evidence shows that if a woman does not over-gain recommended weight, then she will lose that weight as well as (after) a singleton pregnancy.”