July 20, 2010 -- Babies born to women who are overweight or obese are more likely to be delivered prematurely, and the heavier the mother's weight, the greater the risk, a study shows.
But studies examining the impact of maternal body weight on early birth and low birth weight have been mixed, with some finding an increased risk for the complications and others finding no such risk. In a few studies, maternal obesity seemed to be protective against delivering a low-birth-weight baby.
In an effort to better understand the relationship between maternal weight and early deliveries or low-birth-weight deliveries, researchers in Canada pooled data from 84 studies that included more than 1 million women.
The analysis confirmed that overweight and obese women have a higher risk of delivering prior to 32 weeks of gestation and a higher risk of induced delivery before 37 weeks.
"Thirty-two weeks is a really important benchmark in pregnancy," study researcher Sarah D. McDonald, MD, of McMaster University in Hamilton, Ontario, tells WebMD. "Babies born this early are much more likely to be sick and they tend to spend a much longer time in the nursery."
Obesity and Preterm Birth
The analysis initially showed no difference in the overall risk of early delivery (before 37 weeks) associated with body weight. But after adjusting for bias, being overweight or obese was associated with a 30% increased risk for early delivery.
McDonald says the risk for very early delivery (before 32 weeks) and induced early delivery before 37 weeks increased with maternal weight. Compared to normal-weight women, the risk for both outcomes was about 15%, 50% and 80% higher, respectively, among overweight, obese, and very obese women.
She adds it is no surprise induced early deliveries are more common among overweight and obese women, since their risk for pregnancy-related complications is so much greater.
The initial analysis found a slight decrease in risk for delivering low-birth-weight babies among overweight and obese women, but, once again, the effect disappeared when the researchers adjusted for bias.
Guidelines for Weight Gain During Pregnancy
In response to the growing incidence of obesity among women of childbearing age, the Institute of Medicine (IOM) last year revised its guidelines for weight gain during pregnancy for the first time in 19 years.
For the first time, the public health policy group included an upper limit on how much weight women already carrying extra pounds should gain, calling on overweight women to gain no more than 20 to 25 pounds with a single-birth pregnancy.
"The old adage that a pregnant woman is eating for two just isn't true," McDonald says. "You certainly shouldn't be eating twice as much as an adult human needs. You should be eating the right foods and the right amount of food to grow a healthy baby."
In addition to being good for babies, University of Texas Medical Branch chief of obstetrics and maternal-fetal medicine George Saade, MD, tells WebMD that limiting weight gain during pregnancy has important benefits for moms.
He says doctors should consider pregnancy an important "teachable moment" for counseling their female patients about the importance of maintaining a healthy weight.
"One of the most important contributors to long-term obesity in women is gaining a lot of weight during pregnancy and failing to lose it after delivery," he says.
He says women should do all they can to maintain a healthy weight during their childbearing years and beyond.