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Pregnancy More Likely Among Overweight Women Taking Oral Contraceptives Than in Thinner Women, Study Says

KaiserNetwork.org Daily Reproductive Health Report
January 3, 2005

Overweight women are more likely to become pregnant while taking oral contraceptives than their thinner counterparts, according to a study published in the January issue of Obstetrics and Gynecology, the Seattle Post-Intelligencer reports. Epidemiologist Victoria Holt and colleagues from the Fred Hutchinson Cancer Research Center in Seattle studied 781 women who used oral contraceptives between 1998 and 2001, 248 of whom became pregnant while taking the pills and 533 who did not (Davidow, Seattle Post-Intelligencer, 12/29/04). Women in the study with a body mass index -- calculated using each woman's height and weight -- greater than 27.3 were 60% more likely to become pregnant despite taking oral contraceptives than women with BMIs of 27.3 or lower. Women with BMIs greater than 32.2 had a 70% higher risk of pregnancy than women with BMIs of 27.3 or lower, and the increased risk of pregnancy for overweight women who never missed a pill was more than twice that of thinner women who had "perfect pill use," the Seattle Times reports. Although researchers did not examine why the disparities exist, they hypothesized that overweight women's higher metabolic rates and greater number of liver enzymes might explain their increased chance of unplanned pregnancies despite hormonal contraceptive use (Sommerfeld, Seattle Times, 12/29/04). The findings "echo" the results of a 1999 pilot study led by Holt, and other studies have found a higher risk of pregnancy in overweight and obese women who use other types of hormonal contraceptives, according to the Post-Intelligencer (Seattle Post-Intelligencer, 12/29/04).

Reaction, Recommendations
"Overweight and obese women have a significantly higher risk of getting pregnant while on the pill than women of normal weight, and this translates into a substantial number of unplanned pregnancies," Holt said, adding, "This higher risk of pregnancy also translates into a higher number of obesity-related complications of pregnancy, which range from gestational diabetes and high blood pressure to caesarean delivery." However, she said that women who are overweight should not ask for a higher dose of oral contraceptives because hormonal contraceptives are associated with an increased risk of cardiovascular disease and overweight women already are at higher risk for CVD (BBC News, 12/29/04). Ruth Krauss, a gynecologist at Group Health who was not involved in the study, said doctors should mention the increased pregnancy rate to overweight patients. "This study might prompt more frequent use of nonhormonal contraception," including intrauterine devices, Krauss said (Seattle Times, 12/29/04). Holt recommended that overweight women who eventually want to conceive use a backup form of contraception, such as condoms, and women who do not wish to conceive consider sterilization, according to BBC News (BBC News, 12/29/04).

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