The aim of the study is to evaluate the association between gestational diabetes mellitus (GDM) and maternal obesity and weight gain during pregnancy. A prospective cohort study screened 614 consecutive gravid patients for GDM using 50 g glucose challenge test (GCT). The pregnant women were divided into 4 groups according to their prepregnancy body mass index (BMI). Group I, II, III and IV constituted when the BMI < 18.5 kg/m² (n = 16), 18.5-24.9 kg/m² (n = 455), 25-29.9 kg/m² (n = 122), and >30 kg/m² (n = 21) respectively. All the pregnant women were also evaluated in terms of their weight gain during pregnancy and these cases were recruited in 3 groups as low, ideal and high weight gain groups. Overall, a positive 50 g GCT result was identified in 106/614 (17.8%) women. GDM was further diagnosed in 12/614 (1.95%) of subjects. The prevalence of GDM in Group II, III and IV was 1.31%, 3.28% and 9.52% respectively (p < 0.05). The cases of Group II in first and second trimester and Group III only in second trimester showed statistically significant positive results of 50 g GCT when they had excess weight gain compared to the ones whose weight gain were in normal range. Women planning pregnancy should be educated about the disadvantages of obesity, being over-weight and should be advised to have an ideal prepregnancy BMI and ideal weight gain during pregnancy.