Third-trimester HbA1c was investigated as a predictor of diabetes following GDM. After five years, 73/196 (37%) of the women had developed diabetes. HbA1c ≥36 mmol/mol (≥5.4%) was associated with a 5.5-fold increased risk of diabetes. HbA1c showed high specificity but low sensitivity to predict diabetes post-partum. HbA1c could be used as a means of selecting high-risk women for interventions.