The risk factors that included the women's age, the obstetrical history, the educational level, the marriage relation and the habit of contraceptive using were matched for comparing the risks in two groups in order to find out the failed causes. Results: The factors that were a low education, a complex obstetrical history, separation of their partners, wrong using contraceptive pill showed the principle causes of unintended pregnancy (OR 4.4, 2.0, 10.6, respectively).