CONTEXT: Knowing the extent to which contraceptive nonuse, incorrect or inconsistent use, and method failure account for unintended pregnancies ending in abortion, as well as reasons for nonuse and imperfect use, can help policymakers and family planning providers support effective contraceptive use. METHODS: Contraceptive use patterns among a nationally representative sample of 10,683 women receiving abortion services in 2000-2001 were examined, as well as reasons for nonuse, problems with the most frequently used methods and the impact emergency contraceptive pills have had on abortion rates. RESULTS: Forty-six percent of women had not used a contraceptive method in the month they conceived, mainly because of perceived low risk of pregnancy and concerns about contraception (cited by 33% and 32% of nonusers, respectively). The male condom was the most commonly reported method among all women (28%), followed by the pill (14%). Inconsistent method use was the main cause of pregnancy for 49% of condom users and 76% of pill users; 42% of condom users cited condom breakage or slippage as a reason for pregnancy. Substantial proportions of pill and condom users indicated perfect method use (13-14%). As many as 51,000 abortions were averted by use of emergency contraceptive pills in 2000. CONCLUSIONS: Women and men need accurate information about fertility cycles and about the risk of pregnancy when a contraceptive is not used or is used imperfectly. Increased use of emergency contraceptive pills could further reduce levels of unintended pregnancy and abortion.