 Each of the eligible mother-infant pairs will be studied in 3 separate rounds, at 4, 12 and 24 weeks post-partum. In each round, two-week studies of breast milk transfer from the mother to the infant will be carried out, using the deuterium dose-to-the-mother technique. In each round, the women will be requested to attend the Dhaka Hospital of ICDDR, B on day 0, for collection of baseline saliva samples from mother-infant pairs and administration of deuterium to the mothers. Study health workers will make home visits on days 1, 2, 3, 4, 13, and 14 and collect saliva samples from the infants and their mothers on days 1, 2, 13 and 14, of each round. 