Background Little is known about the effect of common mental disorders (CMD) among women in the perinatal period on infant development in low-income countries. The aim of this study was to examine the effect of exposures to maternal symptoms of ante- and post-natal CMD on infant social-emotional development in a low-income setting. Methods A prospective community-based investigation in which a cohort of pregnant women was recruited in rural northern Vietnam and followed until 6 months postpartum. Psychosocial and biological data were collected in four assessment waves. The outcome was 6-month old infants scores on the Bayley Scales of Infant and Toddler Development Social-Emotional Questionnaire. Direct and indirect effects of maternal CMD on the outcome were tested simultaneously with path analysis. Results Complete data were available for 378 mother-infant dyads. There were no direct effects of ante- or post-natal CMD on infant Social-Emotional scores. However, there was an indirect pathway (path coefficient -1.11, 95 CI -1.79 to -0.42) in which antenatal CMD were associated with increased likelihood of postnatal CMD, which were associated with reduced parenting self-efficacy and less affectionate and warm parenting practices, which were associated with lower infant social-emotional scores. Parenting self-efficacy and practices also mediated the adverse effects of a woman being young or of high parity or experiencing poverty, intimate partner violence, a poor relationship with her own mother, non-economic life adversity and insufficient breastmilk, on infant social-emotional development. Limitations We acknowledge some limitations including (1) a moderate rate of attrition, (2) the use of a screening test for perinatal CMD, (3) the Bayley scales are not yet validated for use in Vietnam and (4) possible response bias in which maternal perceptions of their infants were influenced by their mood. Conclusions These data indicate that women s antenatal and postnatal mental health is a crucial but currently inadequately understood determinant of the social and emotional development of infants in low-income settings.