participants were recruited into study from 4 geographically distributed public hospitals in Hong Kong between 2006 and 2007. participants were followed for one year or until they had completed breastfeeding,whichever came first. the study methods and data sources have been described in greater detail elsewhere. Briefly, 1417 breastfeeding mother-infant pairs were recruited in the immediate postnatal period. inclusion criteria were intention to breastfeed, singleton pregnancy,Hong Kong resident, Cantonese speaking, and no serious medical or obstetric complications. Additionally, selection criteria for their infants were a gestational age of at least 37 weeks,an Apgar score of at least 8 at 5 minutes, a birth weight of at least 2500 g, no severe congenital malformation, no placement in the special care nursery after birth for 48 or more hours, and no admission to the neonatal intensive care unit.