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Obese women are at risk for pregnancies exceeding 40 weeks

Research Activities, October 2009

Babies who stay in the womb past the 40-week mark are at risk for serious complications: excessive birth weights, restricted growth, diminished oxygen supply, and death. A new study finds that women who were obese before becoming pregnant ran a high risk of having a pregnancy that went 40 weeks or longer. Additionally, white women and women who had never given birth tended to have pregnancies that crept into the 40-, 41-, and 42-week zone, compared with black, Latina, and Asian women and women who had previously given birth. Women aged 30 to 39 were also more likely than younger women to have pregnancies that reached the 41-week mark.

The authors suggest that reducing obesity through exercise and diet, both before and during pregnancy, can curb the risk of complications that come with long gestations. For example, for every 20 women who decreased their body mass indexes to below the obesity range, 1 fewer woman would go past 41 weeks of gestation. Doctors who treat obese pregnant women should consider separating the amniotic sac from the uterus with a cervical exam or counseling obese women to have intercourse to prompt labor to begin before pregnancies reach the 40-week mark, the authors suggest.

Reasons why obese women carry their babies longer are unknown. One explanation the authors put forth is that endocrine factors that initiate labor may be affected by extra hormones produced by the fat tissue. Eugene Washington, M.D., M.Sc., of the University of California, San Francisco, and colleagues reviewed 119,162 women's birth records from a managed care organization in Northern California from 1995 to 1999. The study was funded in part by the Agency for Healthcare Research and Quality (HS10856).

See "Who is at risk for prolonged and postterm pregnancy?" by Aaron B. Caughey, M.D., Ph.D., Naomi E. Stotland, M.D., Dr. Washington, and Gabriel J. Escobar, M.D., in the June 2009 American Journal of Obstetrics and Gynecology 200(6), pp. 683.e1-683.e5.

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Page last reviewed October 2009
Internet Citation: Obese women are at risk for pregnancies exceeding 40 weeks: Research Activities, October 2009. October 2009. Agency for Healthcare Research and Quality, Rockville, MD.