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Obesity May Increase Risk of Preterm Birth

Study Highlights Risks of Overweight or Obesity During Pregnancy
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WebMD Health News
Reviewed by Laura J. Martin, MD

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.

Obesity is a well-established risk factor for a long list of pregnancy and birth complications, including miscarriage, gestational diabetes, preeclampsia, and cesarean delivery.

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.

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