To understand vitamin D’s role during pregnancy, lactation and early infancy, we must first understand what vitamin D is, and its sources. Vitamin D is a preprohormone that is created in the epidermal layer of the skin following the exposure of 7-dehyrocholesterol to ultraviolet B light, within a specific wavelength between 290-315 nanometers , producing previtamin D. Through a thermal conversion in the skin, the previtamin D is converted into cholecalciferol or vitamin D3 and transported systemically via either vitamin D binding protein or albumin (see Figure 1 below). Once in the liver, vitamin D is converted through a 25-hydroxylation to 25-hydroxy-vitamin D (25(OH)D), where it then circulates throughout the body attached mainly to VDBP or albumin [2, 3]. It is converted by the proximal tubules in the kidney to the active hormone 1,25-dihydroxy-vitamin (1,25(OH)2D) with a hydroxylation at the 1-alpha position that is megalin-mediated . This conversion also is regulated by parathyroid hormone. When there is vitamin D deficiency in the body, circulating PTH increases [2, 3]. Not limited to the kidneys, there are receptors to vitamin D moieties throughout the body, and include immune cells, such as dendritic cells and monocytes/macrophages and lymphocytes [6-10], where 1,25(OH)2D can be synthesized within the nuclear membrane.