(HealthDay)—Pregnancy isn't a license to gain weight, say researchers who have found that heavier moms-to-be tend to have fatter babies at greater risk for serious health issues.
Bigger babies can pose problems during delivery, and a baby that's large for its age is at higher risk for obesity, asthma and diabetes later in life.
"Obesity, excessive weight gain during pregnancy and pregnancy-related diabetes all contribute to having big babies," said lead researcher Shin Kim, of the division of reproductive health at the U.S. Centers for Disease Control and Prevention. And all three are increasing in the United States, she added.
But excessive weight gain carries the greatest risk among those three, according to the report, which was published in the April issue of the journal Obstetrics & Gynecology.
A large baby is one that is at or above the 90th percentile for weight at its gestational age. In the United States, about 9 percent of newborns fall into that category each year.
"One of the biggest concerns is that babies born too big have a higher risk of developing diabetes and obesity in adulthood," Kim said.
A big baby also raises the odds of a cesarean birth, prolonged delivery and excessive birth trauma to the mother, said Dr. Jennifer Wu, an obstetrician and gynecologist at Lenox Hill Hospital in New York City. Wu was not involved in the study.
"In the baby, you can have something called shoulder dystocia, where the head delivers but the shoulders are so broad that they don't come out, and the baby can wind up with permanent injuries," Wu said. "Shoulder dystocia is one of the most serious obstetric emergencies and tends to happen in big babies."
Overweight and obese women who want to get pregnant should get prenatal counseling on nutrition and weight, Kim said. A body-mass index—a measure of body fat based on height and weight—of 25 or higher is considered overweight.
"They should talk to their doctor about entering pregnancy at a healthy weight, and once they are pregnant, talk with their doctor about their weight-gain goals during their pregnancy," Kim said. Combined with monitoring during pregnancy, this can increase the odds of a healthy pregnancy and smooth delivery, Kim and Wu said.
Women need to get over the notion that when they are pregnant they are eating for two, said Dr. Jill Rabin, chief of ambulatory care obstetrics and gynecology at Long Island Jewish Medical Center in New Hyde Park, N.Y.
"We know there is a direct relationship between maternal weight gain and diabetes," Rabin said.
If the mother develops pregnancy-related diabetes, her pancreas is stressed and blood sugar increases and goes into the baby's blood, Rabin said. To compensate for the excess sugar, the baby pumps out insulin, which is a growth hormone, and the baby gets big.
"You should optimize your weight well before you get pregnant so you will have children that deliver easier and be at less risk for obesity and diabetes in their lives," Rabin said.
"If you don't do it for yourself, do it for your children," she added.
For the study, Kim's team collected data on births in Florida from 2004 to 2008. They found that 5.7 percent of women with normal weight gain and no diabetes had large babies.
Among overweight and obese women, 12.6 percent had large babies. For women who gained excess weight during pregnancy, 13.5 percent had large babies, as did 17.3 percent of women who gained excess weight and also had pregnancy-related diabetes.
Kim's group said the prevalence of large babies could be dramatically reduced among women most prone to being overweight or obese.
Among blacks, for instance, incidence of big babies could fall by 61 percent in the absence of obesity, excessive weight during pregnancy and pregnancy-related diabetes, the researchers said.
Among all the women in the study, pregnancy-related diabetes contributed the least to having a large baby—2 percent to 8 percent, depending on race and ethnicity. Gaining excess weight during pregnancy contributed the most—about 22 percent to nearly 38 percent.
Although the study showed an association between a mother's weight gain and increased health risks for her baby, it did not prove cause-and-effect.
Explore further: Overweight and obese women more likely to have large babies
For more on weight during pregnancy, visit the American Pregnancy Association.