Infant growth is determined genetically, and varies according to an infant's nutritional input. An average newborn in the United States weighs 7.5 lb. (3.4 kg), and her weight nearly triples by age 1. Height at birth averages to 20 in. (51 cm), and at age one, reaches approximately 28 in. (71 cm). An infant's head remains disproportionately large compared to the rest of the body. Head circumference at birth is approximately 13-15 in. (33-38 cm) and by 12 months usually increases to approximately to 16-19 in. (41-47 cm). The rate of head circumference growth slows after 12 months.

During the first month the infant's musculature adapts from limitations imposed by the fetal position, to one that allows greater flexion of elbows, hips, and knees. By six weeks, an infant's musculature develops enough that when pulled from a lying position (supine) the baby's head and neck muscles reduce the lack of head...