More than one in five women gain excessive weight during pregnancy and they have a higher risk of delivering a baby who weights 9 pounds or more, according to a news study led by researchers at the Kaiser Permanente Center for Health Research.

Teresa Hillier, MD, senior investigator at the Kaiser Permanente Center for Health Research, Portland, Ore., and her colleagues followed 41,540 women who gave birth in Washington, Oregon and Hawaii from 1995-2003. The researchers used patient medical records and birth certificates to note the mother’s weight gain and the baby’s birth weight.

The study found that women who gained more than 40 pounds during their pregnancies were nearly twice as likely to have a heavy baby. More than 20% of women who gained more than 40 pounds gave birth to heavy babies and less than 12% of those who gained less than 40 pounds had heavy babies. 30 percent of women who gained more than 40 pounds and suffered gestational diabetes had heavy babies, compared to just 13.5% of those with gestational diabetes who gained less than 40 pounds.

Heavier babies are at risk of becoming heavy adults, the researchers said. “A big baby also poses serious risks for both mom and baby at birth – for mothers, vaginal tearing, bleeding, and often C-sections, and for the babies, stuck shoulders and broken collar bones,” said Teresa Hillier, lead-author of the study. Women should avoid excessive weight gain during pregnancy, the study’s authors conclude. Gains of more than 35 to 40 pounds may result in a large-for-gestational age infant.

About three to five pounds is generally gained in the first trimester, but up to 10 pounds can be normal. It is important to note that excessive weight gain during pregnancy makes postpartum loss more difficult.

The study was published in the November issue of the journal Obstetrics & Gynecology.