Results of subgroup analyses by the time of sample collection manifested that, if sampled before 36 weeks of pregnancy, GDM-affected pregnant women had significantly higher levels of circulating FGF21 than healthy expectant mothers (p = 0.009), whereas if sampled at or after 36 weeks of pregnancy, FGF21 levels of the case group and the control group did not significantly differ from each other (p = 0.094). This evidence concerns the gene FGF21 and gestational diabetes.