CONTEXT: Many Nigerian women experience unwanted pregnancies. To prevent associated health problems, it is important to understand the factors related to unwanted pregnancy in Nigeria. METHODS: A community-based survey of 2,978 women aged 15–49 was conducted in eight Nigerian states. Univariate analyses and multivariate logistic regression analyses were used to determine the incidence of unwanted pregnancy, the incidence of seeking an abortion among women with unwanted pregnancies and the factors associated with unwanted pregnancy and abortion-seeking behavior. Additional analyses examined the prevalence of contraceptive use and women’s reasons for seeking to terminate unwanted pregnancies and for not practicing contraception at the time their unwanted pregnancies were conceived. RESULTS: Twenty-eight percent of respondents reported ever having had an unwanted pregnancy; of those, half reported having attempted to end their last unwanted pregnancy. Forty-three percent of women who sought an abortion did so because they were not married, were too young or were still in school. Of the women who were not practicing contraception when they had the unwanted pregnancy, 44% said that they were unaware of family planning, and 22% that they either did not have access to contraceptive services, services were too expensive or they were afraid of side effects. At the time of the survey, 27% of all respondents were at risk of an unwanted pregnancy. Almost half were unaware of contraceptive methods. CONCLUSIONS: Nigerian women often turn to abortion to avoid unwanted births. The provision of family planning counseling and information could substantially reduce the incidence of unwanted pregnancy and induced abortion in Nigeria.