Background
Glycated haemoglobin (HbA1c) is an important tool for assessing glycaemic status in patients with diabetes, but its usefulness in gestational diabetes mellitus (GDM), is unclear.

Aims
The aim of this study is to evaluate whether HbA1c in women with GDM is valuable in predicting adverse pregnancy outcomes.

Materials and methods
A retrospective review of women with GDM who had HbA1c measured at diagnosis of GDM (GHb‐diag) and at 36 weeks gestation (GHb‐36 weeks) was conducted. The association between HbA1c and various pregnancy outcomes was assessed

Results
Among 1244 women with GDM in our cohort, both GHb‐diag and GHb‐36 weeks were independent predictors for large‐for‐gestation (LGA) babies (OR 1.06, P = 0.005 and OR 1.06, P = 0.002, respectively) and neonatal hypoglycaemia (OR 1.10, P < 0.001 and OR 1.09, P < 0.001, respectively). Women with HbA1c ≥ 5.4% (35 mmol/mol) at diagnosis had significantly greater risk for LGA (15.3% vs 8.2%, P < 0.001) and neonatal hypoglycaemia (42.2% vs 23.6%, P < 0.001) than those below this cut‐off. The difference between GHb‐diag and GHb‐36 weeks was small and improvement in HbA1c by 36 weeks was not associated with better pregnancy outcomes. Conclusion We showed that measurement of HbA1c, either at the time of diagnosis of GDM or toward the end of pregnancy, were both associated with adverse pregnancy outcomes. Women with elevated HbA1c (>5.4% or 35 mmol/mol) at diagnosis of GDM should be monitored closely during pregnancy. However, there is not enough evidence to suggest that repeating HbA1c toward the end of pregnancy will provide additional information in predicting adverse pregnancy outcomes.