OBJECTIVE To evaluate the potential role of glycosylated haemoglobin (HbA1C) as an early biomarker for gestational diabetes. METHODS In a retrospective analysis, healthy women who went on to give birth to a singleton newborn in a tertiary medical center in Petach Tikva, Israel, underwent measurement of HbA1C in the first trimester (up to 12 weeks of pregnancy) between August 1, 2007, and December 31, 2014. Women with type 1 or type 2 diabetes, HbA1C ≥6.5%, and/or fasting plasma glucose ≥126 mg/dL were excluded, as were women whose glucose levels had already been tested before 24 weeks of pregnancy. Data were extracted from a maternal and neonatal database. The primary outcome measure was the association between first trimester HbA1C and adverse pregnancy outcome, primarily, gestational diabetes. RESULTS The cohort included 142 women. HbA1C concentration was linearly and inversely correlated to length of gestation (r=-0.317, P<0.001). Higher HbA1C was associated with gestational diabetes. An HbA1C concentration of ≥5.45% predicted gestational diabetes with 83.3% sensitivity, 69% specificity, and gave positive and negative predictive values of 53% and 90.8%, respectively. CONCLUSIONS Early pregnancy HbA1C could serve as a predictor of gestational diabetes. Ideally, HbA1C should be considered in multi-parameter prediction models to enhance accuracy.