The function and requirement of vitamin D during pregnancy for both mother and fetus have remained a mystery. This fact was highlighted by The Cochrane Review in 2000, which reported a lack of randomized controlled trials (RCTs) with respect to vitamin D requirements during pregnancy. Unfortunately, during the past decade only a single RCT has been performed with respect to vitamin D requirements during pregnancy. In this review we will discuss vitamin D metabolism during pregnancy as well as the consequences of vitamin D deficiency on skeletal, nonskeletal, and birth outcomes using birth observational data and data from our recent RCT. New RCT data strongly support previous observational studies in that improving nutritional vitamin D status will improve birth outcomes. The new RCT data indicate that 4,000 IU/day vitamin D(3) during pregnancy will "normalize" vitamin D metabolism and improve birth outcomes including primary cesarean section and comorbidities of pregnancy with no risk of side effects.