Oral contraceptive failures and body weight: findings in a large cohort study.

This report looks at data from 17 032 women who took part in the Oxford Family Planning Association contraceptive study. Thirty-eight first accidental pregnancies occurred during 6779 woman-years of use of progestogen-only oral contraceptives (0.56 per 100) and 95 first accidental pregnancies occurred during 48 692 woman-years of combined oral contraceptive use (0.20 per 100). Although associations with age and parity were found, there was no evidence of any influence of body weight on the risk of accidental pregnancy with either form of oral contraceptive.