 Vitamin D deficiency is common among pregnant and non-pregnant Bangladeshi women. The consequences of vitamin D deficiency during pregnancy are unknown, but increasing evidence suggests it may compromise fetal growth and development of the immune system. We hypothesize that vitamin D deficiency in pregnant women has an adverse effect on the newborns defenses against infectious diseases in early infancy, and thus contributes to the high rates of neonatal mortality in South Asia. Randomized controlled trials of antenatal vitamin D supplementation are needed to test this hypothesis. However, preliminary studies are first necessary to establish a safe and efficacious dose of vitamin D to be used in such trials. In the proposed study, we will measure the response of the 25-hydroxyvitamin D serum concentration a biomarker of vitamin D status to oral vitamin D3 supplementation in pregnant women and non-pregnant women of reproductive age. The goal is to establish a safe, efficacious and feasible weekly vitamin D supplementation regimen for use in future trials. Five groups of approximately 15 women 3 groups of pregnant and 2 groups of non-pregnant women; total up to 75 women will be supplemented and followed closely for 10 weeks non-pregnant women or until delivery pregnant participants. The primary outcomes will be the change in 25-hydroxyvitamin D concentrations following vitamin D supplementation, and safety parameters that reflect calcium regulation serum albumin-adjusted calcium concentrations and urine calcium excretion and fetal development. 