Background: It has long been believed that a lack of vitamin D increases the likelihood of developing glucose intolerance. During pregnancy, pregnant women with gestational diabetes mellitus (GDM), impaired glucose tolerance (IGT), and non-GDM control participants were all compared for their serum levels of 25-hydroxy vitamin D3. Methods: In this case-control study, 111 non-GDM control women were compared to 54 women with diagnosed GDM, 39 women with IGT (1 abnormal oral glucose tolerance test), and 54 women without GDM. In terms of gestational age, age, and body mass index, controls and the IGT and GDM groups were matched. Results: When compared to non-GDM controls, the maternal blood 25-hydroxy vitamin D3 concentration in the GDM and IGT groups at 24-28 weeks of gestation was considerably lower (P =.001). Plasma 25-hydroxy vitamin D3 concentrations were consistent with a diagnosis of vitamin D insufficiency in 83.3% of GDM and 71.2% of the control group (P =.03). When compared to the control group, women with GDM had a 2.66-fold higher probability of being deficient (25-hydroxy vitamin D3 15 ng/mL). Conclusions: According to these findings, vitamin D deficiency is more common in women with IGT/GDM, and more research is needed to determine how vitamin D status affects a woman's ability to tolerate glucose during pregnancy.