In conclusion, in our study’s population of pregnant women, we found that DNA methylation in genes related to vitamin D metabolism differs in the last trimester of pregnancy between women with GDM who were treated with insulin versus diet-treated women and controls, supporting our previously suggested hypothesis that the higher levels of vitamin D observed in women with GDM exposed to insulin, in comparison to women with GDM who were not treated with insulin and controls, could be, at least in part, mediated by epigenetic mechanisms. This evidence concerns the gene INS and gestational diabetes.