Wang M, Chen Z, Hu Y, et al. - Published literature was retrieved and screened from PubMed, Embase, Web of Science, CNKI (China National Knowledge Infrastructure), Wanfang, and Cochrane Center Register of Controlled Trails up to May 2020 to assess the impacts of vitamin D supplementation on glycemic control and maternal-neonatal outcomes in women with established gestational diabetes mellitus (GDM). Nineteen RCTs (1,550 candidates) were eligible for meta-analyses. Overall, vitamin D supplementation significantly decreased serum fasting plasma glucose, insulin concentration and the homeostasis model assessment of insulin resistance in women with GDM. Furthermore, vitamin D supplementation significantly decreased adverse maternal outcomes in pregnant women with GDM, including cesarean section, maternal hospitalization and postpartum hemorrhage. Several adverse neonatal complications including neonatal hyperbilirubinemia, giant children, polyhydramnios, fetal distress and premature delivery have also been significantly reduced. This systematic review and meta-analysis indicated that supplementation of GDM women with vitamin D could lead to an improvement in glycemic control and a decrease in maternal-neonatal adverse outcomes.