There are many psychological and physiological changes during pregnancy and postpartum periods that are sometimes they become pathologic. Thus, it is necessary for a medical team to identify those patients and their families who have a predisposition to mental disorders and to guide them through this period. Aimed at assessing the prevalence and predisposing factors of mental disorders during pregnancy, an analytical-descriptive and cross-sectional study was performed on 267 pregnant women. The data were collected through interview using the General Health Questionnaire and the questionnaire of the predisposing factors. Mental health was assessed with GHQ through the first second and third trimesters of pregnancy, and predisposing factors were assessed with the questionnaire of the predisposing factors in the third trimester. Results showed that the prevalence of mental disorders in the first trimester was 29.7%, in the second trimester 28.6%, and in the third trimester 39.6%. There was a significant correlation between mental disorders and personal, psychological and socioeconomic predisposing factors (P < 0.05). Therefore, taking into account the high prevalence and different predisposing factors of mental disorders, it can be said that mental health care during pregnancy is as important as physical care. Based on the results of this study, susceptible mothers may be defined and identified, and measures may be taken to prevent such disorders.