Contraceptive failure rates and the determinants of failure can be mos
t accurately estimated using prospective data from an area served by a
well-established maternal and child health and family planning progra
m. 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 amon
g 2,856 married women aged 15-49 during the period 1984-1989. Among wo
men 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 substan
tially by contrast the probability of an IUD failure increased, peakin
g at 24 months of use. The injectable maintained a low likelihood of f
ailure regardless of duration of use, and no pattern was evident for o
ther temporary methods. The quality of community health workers' perfo
rmance was associated with the risk of failure of all temporary method
s except the injectable; women's background characteristics associated
with failure varied by method. Calculations from failure rates sugges
t that 25% of births in Bangladesh may reflect contraceptive failure.