Despite widespread concerns about HIV incidence in pregnant and postpartum women there are few data from Africa on HIV acquisition in breastfeeding and subsequent mother-to-child transmission. We measured HIV incidence in a prospective cohort of 413 peripartum and breastfeeding women who tested HIV-negative during pregnancy. In 377 woman-years accrued postpartum (median duration of follow-up, 1 year) there were 7 women infected after delivery (postpartum incidence, 1.86/100PY; 95% CI 0.88-3.89) with transmission to 2/7 (28%) infants.