The chances of having a baby weighing 9 pounds or more are double for pregnant women who gain more than 40 pounds, a study of 41,540 women and babies in Hawaii, Oregon and Washington shows.

The finding was surprising because it is true for all women, whether they have normal blood sugar or gestational diabetes, Dr. Teresa Hillier, endocrinologist and senior investigator at the Kaiser Permanente Center for Health Research in Hawaii and Oregon, said in a telephone interview from Oregon.

More than 20 percent of pregnant women—1 in 5—followed by Kaiser Permanente researchers from 1995 to 2003 gained too much weight, she said.

Pregnant women with excessive weight are more likely to have heavy babies, which poses a serious risk for the mother and baby at birth, she pointed out. Problems could involve vaginal tearing, bleeding and often Caesarean sections for mothers, and stuck shoulders and broken collarbones for babies, Hillier said.

There are also long-term consequences for the baby, who is likely to become a heavy child and adult, she said.

The connection between diabetes during pregnancy and heavier birth weights was known, but this is the first study showing excessive weight gain is a bigger factor, Hillier said, pointing out only 5 percent had gestational diabetes.

Nearly 30 percent of women who gained more than 40 pounds and had gestational diabetes had heavy babies, according to the study. The risk was reduced to 13 percent for women with gestational diabetes who gained less than 40 pounds.

Excess weight also is a potential risk for mothers with gestational diabetes in terms of developing type 2 diabetes after the giving birth, Hillier said.

Fewer than 12 percent of women with normal weight gain had heavy babies, according to the study, funded by an American Diabetes Association grant and published in last month's issue of Obstetrics & Gynecology.

The Institute of Medicine guidelines are under review and likely to be changed, but weight-gain recommendations as of 1990 were that a pregnant woman starting with normal weight should gain no more than 25 to 35 pounds; an overweight woman, 15 to 25 pounds; and an underweight woman, from 28 to 40 pounds, Hillier said.

“;So, more than 40 pounds is too much for any woman regardless where she starts in any pregnancy,”; she said. “;The message applies to all women. It's one more reason to have healthy weight gain and for providers to do a better job counseling women.”;

Healthy weight gain should be the focus, with pregnant women following a nutritious diet “;to get good value for the calories,”; she said. “;Pregnancy is a unique opportunity for healthy nutrition to benefit two people.”;

Hillier said she hopes results of the study will increase awareness among physicians about the importance of counseling pregnant women on weight gain.

Mark Schmidt and Judith Mullen of the Kaiser Permanente Center for Health Research in Hawaii were among the authors of the study.