The overall objective of the proposal is to evaluate the ability of ultrasound to predict and routinely (weekly) monitor changes in the body composition of healthy preterm infants in the neonatal intensive care unit (NICU) in response to nutritional intake. The investigators hypothesize that ultrasound is an accurate method to routinely monitor the quality of weight gain and assess the adequacy of nutritional provision for premature infants in the neonatal intensive care unit.