Add another line to the long list of medical problems associated with obesity: A new study finds that women who are overweight during pregnancy tend to have much heavier babies than normal-weight moms.
"If the baby is born too large it increases the risk for very serious consequences both during delivery, for the mother and the infant, as well as later in life -- for the infant," said study author Mary Helen Black, a biostatistician with Kaiser Permanente Southern California's department of research and evaluation. "There may be a general perception that, 'Oh, the baby's big, but so what?' That's a misperception."
In the new study, published in the journal Diabetes Care on Tuesday, Black and her colleagues examined health records for more than 9,800 women who delivered their babies at a Kaiser Permanente medical center in California between 2005 and 2010. About 60 percent of the women were overweight or obese, and nearly 20 percent developed gestational diabetes.
Compared to healthy-weight women, overweight moms without gestational diabetes were 65 percent more likely to have babies with a birth weight in the 90th percentile or higher. (Women were defined as overweight if they had a body mass index -- a measure of height relative to weight -- of 25 to 30. They were obese if they had a body mass index of 30 or more.)
Obese women without gestational diabetes were 163 percent more likely to have overly large babies.
Because researchers relied on electronic health records for the study, they did not directly measure babies' bodies. But Black said most of the overly large infants were not just tall, and thus proportional to their height. Instead, they probably had too much fat, she said.
Notably, the chances of having a big baby increased regardless of whether moms had gestational diabetes, an established risk factor for greater infant birth weight. Women with the condition have extra glucose in their bloodstream which can cross the placenta, giving babies more sugar than they need and leading to excess growth.
"That's been the thought with why moms with diabetes have larger babies," said Dr. Sharon Herring, an assistant professor of medicine and public health with the Temple University School of Medicine in Pennsylvania, who did not work on the study. "The question was, 'Can women have bigger babies independent of having gestational diabetes?'"
The study shows maternal obesity alone is a risk factor for excess infant weight, said Herring.
"This suggests that there may be other nutrients that are crossing the placenta in overabundance leading to some changes in the physiology and metabolism of the baby," she said.
The study also highlighted the many problems excess infant weight causes. Women with big babies are more likely to have a cesarean section or to hemorrhage during birth. Bigger babies also have a greater chance of getting stuck in the birth canal during delivery.
In the long-term, heavy babies have a greater risk of being overweight as children and adults. In general, obesity increases a person's risk for Type 2 diabetes, high blood pressure, heart disease and respiratory problems.
According to Black, overweight and obese women who are considering getting pregnant should talk to their doctors about steps they can take to get closer to a healthy weight. In the U.S., two out of three women of reproductive age are overweight and one in four are obese, the March of Dime reports.
"If you're already pregnant, you have to really, very seriously monitor your weight-gain over your pregnancy," Black said. "I wouldn't use the word 'diet,' because you need a certain nutrition and caloric intake for the fetus to grow."
Black recommended women follow the Institute of Medicine's guidelines for weight gain during pregnancy. They state that overweight women should gain no more than 25 pounds, while women who are obese should gain no more than 20.
"I think parents all want to give their babies the best start in life possible," Black said. "Women who are thinking about starting a family should really consult with their health care provider to find out if they're in a healthy body weight range."