Abstract: The study investigated the causes and effects of unwanted pregnancy among adolescents in South West Nigeria. A total of 600 secondary school students were chosen as subjects from 6 South Western states of Nigeria using a multistage sampling technique. A purposely structured questionnaire was used as the tool for collecting data from the respondents. Research hypotheses formulated were analysed using inferential statistics of Chi-square (χ²) at α = 0.05 level of significance. The findings were that unprotected sexual intercourse and poor parental care are the causes of unwanted pregnancy while termination of educational programme and medical complications for both the mothers and children are the effects of unwanted pregnancy. Abstinence and adequate teaching of sex education have been seen as measures to prevent unwanted pregnancy among the adolescents. It was recommended among others that sex education should taught as a subject at the secondary level and government should enact laws to compel parents to take proper care of their children to avoid all forms of immorality among the adolescents.