Overweight women who use oral contraceptives have up to a 60% higher risk of becoming pregnant, when compared to women of normal weight, say researchers from the University of Washington School of Public Health in a study published in Obstetrics and Gynecology.

More than 600 women who had used oral contraceptives completed dietary questionnaires between 1990 and 1994. During an interview, the women noted whether they had become pregnant while using oral contraceptives. The results? The heaviest women in the study (those who weighed 70.5 kilograms, about 155 pounds, or more) had a significantly higher risk of becoming pregnant. The heaviest women in the study were at particularly higher risk of becoming pregnant if they took very low-dose or low-dose birth control pills.

According to this study, excess body weight may compromise the effectiveness of oral contraceptives. Your gynecologist can help you determine the best dosage of oral contraceptives for your body size. If you are concerned that your oral contraceptives may not be effective, talk to your doctor and use an additional form of birth control, such as a condom, to prevent pregnancy.

Sources:  Holt VL, Cushing-Haugen KL, Daling JR. Body weight and risk of oral contraceptive failure. Obstetrics & Gynecology May 2002;99(5):820-7.