HIGH BODY MASS INDEX MAY INCREASE RISK OF ORAL CONTRACEPTIVE FAILURE

High body mass index (BMI) is associated with an increased risk of oral contraceptive (OC) failure, according to the results of a case-control study published in the January issue of Obstetrics & Gynecology.

This finding is also consistent with those in studies of contraceptive patches.

The investigators compared 248 health maintenance organization enrollees who became pregnant while using OCs between 1998 and 2001 with 533 age-matched enrollees who were nonpregnant OC users during the same period, according to Medscape. To estimate the risk of pregnancy based on BMI and weight quartile, the investigators calculated adjusted odds ratios using logistic regression.

Compared with women having a BMI of 27.3 kg/m2 or less, the risk of pregnancy while using OCs was nearly 60 percent higher in women with BMI greater than 27.3 kg/m2 and more than 70 percent higher in women with BMI greater than 32.2 kg/m2. Consistent users of OCs, defined as women who missed no pills in the reference month, had more than twice the risk of pregnancy when BMI was greater than 27.3 kg/m2 or greater than 32.2 kg/m2. The risk of pregnancy was more than 70 percent higher in consistent OC users weighing more than 74.8 kg and nearly doubled in women weighing more than 86.2 kg compared with women weighing 74.8 kg or less.

Study limitations include case ascertainment based only on pregnancy information known to Group Health Cooperative, potential bias if women who participated differed in BMI and weight from nonparticipants, and reliance on participant self-report to determine consistency of OC use during the reference month.

"Our results suggest that being overweight may increase the risk of becoming pregnant while using OCs," the authors write. "If causal, this association translates to an additional 2-4 pregnancies per 100 woman-years of use among overweight women, for whom consideration of additional or effective alternative contraceptive methods may be warranted."