Pregnancy represents a time of rapid bodily change, which include physical proportions, physiology and responsibility. Arguably, nothing during these times changes more than the requirement and metabolism of vitamin D, although from current Institute of Medicine (IOM) recommendations one would never surmise this fact.  Do the current recommendations for the requirements of vitamin D during these critical time periods reflect emerging data?  Sadly, no.  The minimal recommendations by the IOM and the zero recommendation by The World Health Organization (WHO), which was echoed by a recent Cochrane Review, stating that there are simply no requirement for vitamin D during pregnancy, are contrary to expanding published data (later reviewed in this chapter) that suggest otherwise. Why these recommendations continue to be made remains unclear, but clearly not recognizing a problem exists suggests that one does not need to address the “problem.”

During these dramatic times of physiologic change, the roles of vitamin D in the pregnant versus, for example, the lactating woman are quite different.  In the pregnant women, we believe the primary role of vitamin D to be an immunomodulatory – rather than a calcium-regulating factor, although, it would also retain that function.  Further, vitamin D inadequacy in early life is clearly an instance of the “Barker Hypothesis”.  This theory states that certain adult-onset diseases might have their roots in nutritional insults sustained in the perinatal period (either in utero or in the early months of infancy or both).  Clearly, conditions associated with vitamin D deficiency such as asthma, multiple sclerosis and other neurological disorders would qualify.

If one is reading this review to relive the past 50 years of vitamin D recommendations during pregnancy, we suggest you read other recent publications for history in this area.  However, if the reader is interested in gaining new insight into the vitamin D requirements and function during pregnancy supported by recent data, we suggest you read on. Also, with regard to randomized controlled trials (RCT’s) for vitamin D, they are largely doomed to fail.  The reasons for this are many and specific cases of this failure will be presented.