 This study will enroll 50 healthy, full-term, newborn infants 37 weeks gestational age by ultrasound and their mothers if they plan to breast-feed exclusively for at least 4 months and also to continue breast feeding for at least 12 months. The infants will be examined, weighed and measured within 48 hours of birth, half of whom will be AGA BW2800g and the other half SGA BW2500g. Infants with any acute or chronic illnesses, or congenital anomalies will not be eligible. Any mother with toxaemia, pregnancy-induced hypertension, multiple pregnancies, previously diagnosed as positive for HIVhepatitis B infections, or habituated to smoking or alcohol consumption will also be excluded. The participants will be recruited from Nandipara, Dhaka, an urban community where the ICDDRB has ongoing clinical study facilities. 