Vitamin D insufficiency and deficiency are prevalent worldwide, and pregnant women and infants are at highest risk for having deficiency and related consequences. Epidemiologic studies have shown an association between vitamin D deficiency and pregnancy outcomes such as preeclampsia, and vitamin D deficiency has been associated with asthma development. We have hypothesized that vitamin D deficiency has contributed to the asthma and allergy epidemic in Western countries. We have conducted the first clinical trial of vitamin D supplementation in early pregnancy – VDAART, Vitamin D Antenatal Asthma Reduction Trial. We will discuss first the effect of vitamin D supplementation on asthma and recurrent wheeze outcomes in the 3-year old offspring. We will then discuss our findings of the effect of vitamin D on the occurrence of pre-eclampsia, including results of the gene expression analysis in a subset of the participants. These results will highlight the importance of adequate vitamin D status in pregnancy and in the prenatal programming of childhood outcomes.