Using induced abortion to measure contraceptive efficacy.

Data from a 1989-1990 case-control study of contraceptive efficacy in Norway compare contraceptive use among women who requested an abortion (1,386 cases) with women who responded to a general fertility survey (2,627 controls). In a logistic regression analysis measuring contraceptive efficacy as the odds of avoiding a pregnancy that terminated in an induced abortion compared with the odds for nonuse, consistent condom use was found to lower fecundity by 88.9%, diaphragm use by 89.3%, the pill by 97.8%, the IUD by 97.6%, vasectomy by 99.5%, and female sterilization by 99.8%. The relative contraceptive efficacy of the condom, the IUD and the pill did not vary by marital status or parity but did vary with age; the proportion by which each of these methods reduced fecundity declined among successively older age-groups.