NEW YORK (Reuters Health) - Among pregnant women at higher-than- average risk of premature delivery, those who are overweight or obese seem to be afforded some protection, a new study suggests.

Researchers found that of 253 women who were followed during pregnancy, those who were overweight or obese had a far lower rate of preterm delivery compared with normal-weight or underweight women. Just over 8 percent gave birth before the 35th week of pregnancy, versus nearly 22 percent of normal-weight or underweight women.

All of the women in the study were considered to be at higher-than-normal risk of preterm delivery because of previous preterm births or bleeding problems during the current pregnancy. But excess weight seemed to lower this risk, the researchers report in the journal Obstetrics & Gynecology.

The reason may be related to differences in the frequency of uterine contractions, according to the investigators, led by Dr. Hugh M. Ehrenberg of Ohio State University Medical Center in Columbus.

They found that normal-weight and underweight women tended to have more contractions between the 22nd and 34th weeks of pregnancy.

The explanation for this finding is not clear, but may have to do with the fact that body fat affects hormone levels, which may in turn affect the frequency of uterine contractions during pregnancy, the researchers note.

Whatever the reason, they conclude, the findings suggest that when doctors screen women for their risk of preterm birth, overweight women who show uterine-contraction patterns similar to those of thinner women may need to be considered high-risk for early delivery.

SOURCE: Obstetrics & Gynecology, January 2009.

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