AIM: Fetal exposure to gestational diabetes mellitus (GDM) is said to alter fetal growth and increase the risk of macrosomia. However, little research on GDM exists in African populations. This study aimed to assess longitudinal fetal growth and neonatal birth measures among Black African babies exposed to GDM. METHODS: Pregnant women (Soweto, South Africa) enrolled into a cohort study were followed up with repeated fetal ultrasounds. At 24-28 weeks' gestation a 2-h 75 g oral glucose tolerance test was performed and GDM was diagnosed using the World Health Organization's 2013 criteria. Neonatal birth measures were assessed. RESULTS: The study involved 741 women; 83 (11.2%) with GDM and 658 (88.8%) without. A total of 4040 fetal ultrasounds were performed. GDM exposure was associated with an increase in fetal growth measures, especially abdominal circumference, which was already seen at 16-18 weeks' gestation. Male fetuses in particular, showed a significant association between GDM exposure and increased abdominal circumference (P = 0.009). Most women with GDM (66.3%) received management; all received diet therapy and 32.7% were prescribed medication. There was no difference in birth measures between the GDM-exposed and unexposed neonates. CONCLUSION: Repeated ultrasound measures identified the effects of GDM as early as 16-18 weeks' gestation, well before a diagnosis of GDM would usually be made. Sex differences in fetal growth were observed, with GDM-exposed male fetuses being more affected with larger abdominal circumferences than females. A low rate of macrosomia was observed compared with historical GDM populations.