The vitamin D status of mother and newborn is directly correlated (1) and low in many countries and cultures around the world (2–7). The greatest amount of growth and development occur in utero and during the first year of life. The human fetus accumulates 30 g of calcium while in utero, 80% of which is accumulated in the 3rd trimester (8, 9). Although vitamin D deficiency can be reversed in human infants through supplementation, it is unclear if deficiency in utero and neonatally continues to manifest as low bone mass and altered bone metabolism despite correction of vitamin D status. Low circulating levels of vitamin D during late pregnancy associate with lower whole body and lumbar spine bone mineral density (BMD)5 in children at the age of 9 y (10). Postnatal vitamin D supplementation is also positively associated with higher bone mass in prepubertal girls (11). The difficulty in longitudinal human studies is that they are confounded by other fetal and postnatal exposures than vitamin D alone and thus the relationship between vitamin D status and bone is unclear.