INS and Insulin resistance: The current study showed a negative non-significant correlation between the vitamin D concentration and GTT values (fasting glucose, at 1 and 2 h) which is in alignment with the findings of Maghbooli et al. confirming the association of poor vitamin D status and the risk of GDM through a negative correlation between serum vitamin D and fasting plasma glucose, fasting insulin, and insulin resistance (calculated by HOMA-IR) (34).