Women who gain more than 40 pounds (18 kg ) during pregnancy have nearly twice the risk of delivering a heavy baby as those who gain less, U.S. researchers said on Friday.

Women who gain more than 40 pounds (18 kg ) during pregnancy have nearly twice the risk of delivering a heavy baby as those who gain less, US researchers said on Friday.

The study of more than 40,000 U.S. women and their babies found as many as one in five women gains too much weight during pregnancy, doubling the chances her baby will weigh 9 pounds (4 kg) or more.

And they found women who gain more than 40 pounds during pregnancy are more likely to have a heavy baby even if they do not have gestational diabetes, a short-term form of diabetes linked with pregnancy that is known to increase the risk of having a big baby.

"Because there are so many women who are gaining more than 40 pounds during pregnancy, it's an important health message for most women to avoid excessive weight during pregnancy," Dr. Teresa Hillier of Kaiser Permanente Center for Health Research in Portland, Oregon, whose study appears in the journal Obstetrics & Gynecology, said in a telephone interview.

Hillier said gaining extra weight during pregnancy increases the risk for having heavy babies, and studies suggest these babies are programed to become overweight or obese later in life.

According to the American College of Obstetricians and Gynecologists, babies who weigh more than 9 pounds at birth are considered heavy.

A large baby can pose risks for a difficult delivery -- increasing the chances of vaginal tearing, bleeding, and Caesarian-sections for the mother and the risk of stuck shoulders and broken collar bones for the baby.

In the study, Hillier and colleagues examined the medical records of 41,540 women who gave birth in Washington, Oregon and Hawaii from 1995-2003. All had been tested for gestational diabetes and 5.4 percent were treated for it with a program of diet, exercise and insulin, if needed, to control blood sugar.

Overall, 20 percent of the women in the study who gained more than 40 pounds -- the upper limit of pregnancy weight gain recommendations in the United States -- gave birth to heavy babies.

Fewer than 12 percent of women in the study with normal weight gain delivered heavy babies, she said.

The group at greatest risk were those who gained more than 40 pounds and had gestational diabetes. Nearly 30 percent of these women had heavy babies, compared with 13.5 percent of women with gestational diabetes who had normal weight gains during pregnancy.

The researchers said the findings suggest all women should avoid excessive weight gain during pregnancy. And women who are being treated for gestational diabetes should also strive to keep weight gain below 40 pounds.

"You can't treat the glucose and ignore the weight gain issue," Hillier said.

Weight gain during pregnancy has been rising over the past two decades, and some researchers suspect this may be fueling an epidemic of childhood obesity.