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Pregnancy risk higher for overweight oral contraceptive users

Obesity may increase the risk of becoming pregnant while using oral contraceptives, say US researchers.

They compared the body mass index of 248 women who became pregnant while using oral contraceptives between 1998 and 2001 with those of 533 women who used oral contraceptives but did not become pregnant.

The researchers found that the risk of pregnancy was nearly 60 per cent higher in women with a BMI greater than 27.3 compared with those having a BMI of 27.3 or less (odds ratio 1.58, 95 per cent confidence interval 1.11–2.24). This rose to over 70 per cent for women with a BMI greater than 32.2 (OR 1.72, CI 1.04–2.82).

The researchers suggest several mechanisms that might explain the relationship. First, they say that higher body weight is associated with increased basal metabolic rate, which may shorten the duration of action of oral contraceptives.

Second, obesity is associated with increased clearance of drugs that undergo phase II hepatic metabolism. A third possibility is that lipophilic oral contraceptives may be absorbed into fat tissue making serum hormone levels lower in overweight women.

The researchers estimate that a 60 per cent increase in pregnancy risk for overweight women would translate to between two and four additional pregnancies per 100 years of oral contraceptive use compared with normal-weight women. They say that consideration should be give to the use of additional or alternative contraception methods for these women (Obstetrics & Gynecology 2005;105:46).

Citation: The Pharmaceutical Journal URI: 10018189

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