Objective: The polymorphisms of peroxisome proliferator–activator receptor-g2 (PPAR-g2) have been suggested to affect glucose metabolism and weight gain. Both conditions show great variations during pregnancy that makes pregnancy a suitable condition to detect any metabolic abnormalities related to PPAR-g2 polymorphisms. The objective of this study is to investigate the prevalence and metabolic impacts of PPAR-g2 polymorphism in control pregnant women and in patients with gestational diabetes mellitus (GDM). Methods: In this case-control study, anthropometric and metabolic variables of 100 non-diabetic pregnant women and of 62 women who were diagnosed as having GDM according to 100 g oral glucose tolerance test (OGTT) were compared on the basis of PPAR-g2 polymorphism by univariate analysis of covariance. Results: There were no statistically significant differences in baseline characteristics and the mean 50 g glucose challenge test values of pregnant women in both groups on the basis of PPAR-g2 genotype, although patients with Pro12Ala polymorphism were significantly taller in GDM group. The Pro12Ala polymorphism had no effect on 100 g OGTT results of patients with GDM. However, patients with GDM who had Pro12Ala polymorphism gained significantly more weight during their pregnancy. Conclusion: The PPAR-g2 Pro12Ala polymorphism was observed to have no effect on glucose metabolism in normal pregnant women and women with GDM. However, only the patients with GDM who had this polymorphism gained significantly more weight during their pregnancy. It seems that Pro12Ala polymorphism plays a dynamic and interactive role in the regulation of BMI and glucose homeostasis.