Women pregnant with twins should be sure to gain the recommended amount of weight, according to a new study, which shows that gaining less weight than recommended during a twin pregnancy ups the risk of early birth and low weight babies.

“A woman should gain about a pound a week; less than that, and we had smaller babies and more pre-term births,” Dr. Nathan Fox of Maternal-Fetal Medicine Associates of New York City told Reuters Health.

Fox and his partners wanted to know if the twin pregnancy weight gain guidelines updated by the Institute of Medicine in 2009 made a difference in pregnancy outcomes.

These recommendations suggest a range of weight gain depending on a woman’s weight at the start of her twin pregnancy. A normal weight woman should gain 37 to 54 pounds; an overweight woman, 31 to 50 pounds; and an obese woman, 25 to 42 pounds.

In looking at 281 mostly normal weight women pregnant with twins, Fox and colleagues found that women who gained the recommended amount of weight did much better in regards to the pregnancy outcomes.

Women who gained less than the recommended amount of weight were more likely to give birth early (before 32 weeks of pregnancy) than women who gained the correct amount of weight (14 percent versus 5 percent). They were also more likely to give birth to smaller babies.

Unlike in singleton pregnancies, gaining enough weight in twin pregnancies can be Herculean task. “It’s often not easy,” Fox said. “We send our patients to nutritionists and have them on high calorie diets a lot to have them gain the right amount of weight. It’s not easy for many (women).”

About 3 in every 100 pregnancies is a twin pregnancy, the investigators note, and preterm births occur about 60 percent of the time, making the issue of correct weight gain even more important.

The strong link between inadequate weight gain in twin pregnancies and increased rates of pre-term birth “has not been conclusively demonstrated previously,” the investigators point out in the journal Obstetrics & Gynecology.

Their findings, they say, shed light on the importance on what “could be a correctable cause of prematurity in twins,” they conclude.

SOURCE: Obstetrics & Gynecology, July 2010.