Abstract. Objective: The aim of this study was to prospectively evaluate the relationship between common mental disorders (CMD) during the antenatal and the postpartum period and breastfeeding for less than four months (BF <4). Methods: A prospective cohort study conducted between May 2005 and March 2007 included 701 pregnant women recruited from primary care clinics of the public sector in São Paulo, Brazil. The followings instruments were in effect: antenatal and postnatal CMD were measured with the Self Report Questionnaire (SRQ-20); social support was assessed with a Brazilian version of the scale used in the Medical Outcomes Study, and questionnaires were employed to obtain socio-demographic, socioeconomic, obstetric, and breastfeeding information. The primary outcome was breastfeeding for BF <4. Crude and adjusted risk ratios (RR), with 95% confidence intervals (95% CI), were calculated using Poisson regression to examine the associations between BF <4 and CMD. Results: BF <4 occurred in 22.8% of the cohort. In the univariate analysis BF <4 was associated with lower maternal age, not having a partner, lower wealth score, having a previous miscarriage, newborn oral tracheal intubation, and CMD in the antenatal and postpartum period. In the multivariate analysis, the following variables were independently associated with BF <4: maternal age (RR: 0.62 (95% CI: 0.39–0.98), newborn oral tracheal intubation (RR: 1.85 (95% CI:1.28–2.65) and CMD during both the antenatal and postpartum (RR: 1.38 (95% CI: 0.98 : 1.95) periods. Conclusion: Among women who have a CMD both during the antenatal and postpartum period, there is a decreased likelihood for breastfeeding longer than 4 months.