Gestational diabetes (GDM) is increasing in prevalence with effects starting in-utero, leading to excessive fetal growth. It is the leading cause of many perinatal complications. The aim was to determine the rate of obstetric and neonatal complications in pregnant women with high fetal weight and a recent diagnosis of GDM during the third trimester, despite normal earlier glycemic control. Prospective cohort study over four years involving pregnant women regularly visiting a single center who had normal glycemic index at 24-28 gestational weeks and ultrasonography (US) suggested high fetal weight during the third trimester. Oral glucose tolerance test was given, dividing the sample into the late GDM (LGDM) and the non-LGDM group. Of 176 women, 24 (13.64%) had LGDM, and 152 (86.36%) had non-LGDM. After exclusions these groups' sizes were (n=21) in LGDM and (n=132) in non-LGDM. Hemoglobin A1c level was significantly higher in LGDM than non-LGDM (5.9% versus 5.1%). However, obstetric and neonatal complications were largely comparable (p≥0.05) but higher in LGDM than non-LGDM women. Exceptions to this were birth weight (3219 g versus 3326 g), large for gestational age at delivery (85.72% versus 88.64%), and gestational age at delivery (37.9 versus 38.2 weeks) in the LGDM vs. non-LGDM groups, respectively. There was a significantly higher cesarean section (CS) rate (76.19% versus 51.52%; p<0.05) in the LGDM group. The rate of newly diagnosed LGDM in pregnant women with high fetal weight during the third trimester by US was 13.64%. They had comparable obstetric and neonatal complications with non-GDM women, except for the rate of CS that was significantly higher in LGDM women.