Abstract: Vitamin D has a wide range of physiological effects. Many animal and human studies have provided evidence that vitamin D plays an important role in insulin sensitivity and pancreatic ?-cell function. This review summarizes the metabolism and mechanism of vitamin D in pregnancy, also provides an overview of studies available on the relationship between vitamin D and gestational diabetes mellitus (GDM). Conflicting results exists on the role of maternal vitamin D status in GDM currently. Hence, well-designed, placebo-controlled, randomized intervention studies are required to establish a true protective influence of vitamin D on glucose homeostasis in GDM.