Researchers at the Kaiser Permanente Center for Health Research, Portland, Ore. report there’s a strong connection between the weight gained during pregnancy and the weight of the baby at birth. More exactly, they found that gaining 40 pounds or more during pregnancy almost doubles the risk of having a baby who weighs 9 pounds or more, thus increasing the health risks to mother and baby.

Also, they say that these women gaining too much weight have more chances of developing gestational diabetes, a form of diabetes often met during pregnancy, which also increases the chances of having heavier babies.

“More than one in five women gain too much weight during pregnancy and only 5% have gestational diabetes,” says Teresa Hillier, MD, senior investigator at the Kaiser Permanente Center for Health Research and the study's lead author.

For the study she and her colleagues followed 41,540 women who gave birth to singleton babies in Washington, Oregon and Hawaii from 1995 through 2003. They used patient medical records and birth certificates to not the mother’s weight gain and the baby’s birth weight.

The researchers looked specifically how many women gained more than 40 pounds, which is the maximum recommended weight gain, and whether their babies weighed more than 9 pounds at birth. According to the American College of Obstetricians and Gynecologists, babies who weigh more than 9 pounds at birth are considered heavy.

Dr. Hillier said that heavier babies are more likely to become heavy adults. Also heavier babies make it more likely the mother will have to deliver by C-section, among other increased health risks.

The study concludes that women should be careful about their weight during pregnancy and aim for a gain of just five to seven pounds in the first 20 weeks, then about 20 to 30 in the remaining weeks.

"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. Hillier said.

The study appears in the journal Obstetrics and Gynecology.