PURPOSE OF REVIEW: Vitamin D status (which is involved in glucose homeostasis) is related to gestational diabetes mellitus (GDM). GDM is characterized by increased resistance to and impaired secretion of insulin and results in higher risk of adverse pregnancy outcomes including operative delivery, macrosomia, shoulder dystocia and neonatal hypoglycemia. Women with GDM and their babies are at increased risk for developing type II diabetes. RECENT FINDINGS: International definitions of vitamin D deficiency and normality are inconsistent. Vitamin D deficiency is common in pregnant women particularly those with poor diets and who have dark skins living in temperate climes with lack of exposure to sunlight. SUMMARY: Few interventional studies indicate that supplementation optimizes maternal vitamin D status or improves maternal glucose metabolism. Observational studies about maternal vitamin D status and risk of GDM are conflicting. This could be because of measurement of vitamin D or differences in population characteristics such as ethnicity, geographic location, gestational age at sampling and diagnostic criteria for GDM. Good-quality randomized controlled trials are required to determine whether vitamin D supplementation decreases the risk of GDM or improves glucose tolerance in diabetic women.