Contraceptive failure rates and the determinants of failure can be most accurately estimated using prospective data from an area served by a well-established maternal and child health and family planning program. In Matlab, Bangladesh, the cumulative probability of contraceptive failure within one year of method acceptance was 1% for the injectable, 3% for the IUD and 15% for the pill and other temporary methods among 2,856 married women aged 15-49 during the period 1984-1989. Among women using no method, the 12-month cumulative probability of conception was 38%. For the pill, the likelihood of failure was consistently high during the first 12-18 months of use, after which it declined substantially; by contrast, the probability of an IUD failure increased, peaking at 24 months of use. The injectable maintained a low likelihood of failure regardless of duration of use, and no pattern was evident for other temporary methods. The quality of community health workers' perf ormance was associated with the risk of failure of all temporary methods except the injectable; women's background characteristics associated with failure varied by method. Calculations from failure rates suggest that 25% of births in Bangladesh may reflect contraceptive failure.

(International Family Planning Perspectives, 22:21-25, 1996)