Aug 7, 2007
The relevant birth defects include missing limbs and malformed hearts.
Smaller scale studies in the past have suggested this link, but this is said to be the largest and most comprehensive study to date.
University of Texas researchers interviewed more than 15,000 new mothers over a five-year period.
The study appears in the journal Archives of Pediatrics and Adolescent Medicine.
Obese women are more likely to have chronic diseases, fertility problems, miscarriages and complications during pregnancy.
But the scientists who carried out this research say this is the strongest link so far between obesity at the time of conception and some birth defects.
The University of Texas team interviewed more than 10,000 new mothers who had babies with birth defects.
The women were asked for their height and weight at the time they conceived.
Another group of 5,000 women with healthy babies was also assessed.
The results suggested that seven different types of birth defect were more common when the mother was obese.
They included spina bifida, heart defects, some genital and bowel abnormalities and small or missing toes, fingers, arms or legs.
Researchers stress the risk of having a baby with birth defects is low, even for obese women.
Amongst women of healthy weight, about three in 100 babies will have serious birth defects. That seems to rise to about four in 100 for obese mothers.
The researchers are not sure how to explain their findings.
The defects may be a direct result of obesity but could equally relate to other factors, like diet.
Those questions will be addressed by further research.
Professor Nick Wald at The UK's Wolfson Institute of Preventative Medicine said the defects may not be related to the actual weight of the mother.
Incidents of spina bifida, a condition which occurs when part of the spinal cord is uncovered, have been greatly reduced by better nutrition and in particular the addition of folic acid to pregnant women's diet.
"The women in this study may not be getting adequate nutritional intake," he said.
"And while they have tried to exclude diabetics, there may be many cases of Type 2 which have gone undetected, and this has long been known to pose a risk in pregnancy."
Professor Michael Patton, the medical director of charity BDF (Birth Defects Foundation) Newlife, said: "At present there are no increased concerns for mothers who are overweight but the best advice is still to eat a healthy balanced diet before and during pregnancy."
The World Health Organization says global obesity has now reached epidemic proportions.
It estimates that more than one billion adults are overweight, about a third of them clinically obese.