In New Zealand, haemoglobin A1c measurements are routinely offered at booking, preferably before 20 weeks gestation, to detect pre‐existing hyperglycaemia. A haemoglobin A1c <5.9% (41 mmol/mol) is considered normal based on the reference range for the non‐pregnant population. Aims To determine pregnancy‐specific haemoglobin A1c centiles by gestation and ethnicity. Materials and Methods This is a population‐based observational study of pregnancies uncomplicated by diabetes (pre‐existing or gestational) with ≥1 haemoglobin A1c measurement. Haemoglobin A1c centiles were calculated from data extracted from electronic laboratory and clinical records for pregnancies during 2008–2010. Results Included were 6800 pregnancies, European 80% (5462), Māori 6% (415), Pacific Islander 3% (196) and 11% (727) ‘Others’ (mostly Asian). Haemoglobin A1c levels fell with increasing gestation, reaching a nadir at 24 weeks, a trend verified by longitudinal data from 112 women. The 97.5th centile for haemoglobin A1c in European women was 5.76% (39.5 mmol/mol) at 8+0 weeks, 5.70% (38.8 mmol/mol) at 16+0 weeks, and 5.65% (38.3 mmol/mol) at 24+0 weeks. Non‐European women had both higher plasma glucose levels (although within the range considered normal) and higher mean haemoglobin A1c levels compared with Europeans; mean (SD) difference in haemoglobin A1c in Māori +0.13% (0.05) (+1.4 mmol/mol (0.5)), Pacific +0.20% (0.03) (+2.2 mmol/mol (0.3)), ‘Others’ +0.10% (0.03) (+1.1 mmol/mol (0.3)). Conclusions The New Zealand haemoglobin A1c cut‐point ≥5.9% (41 mmol/mol) for identifying hyperglycaemia in early pregnancy is greater than the 97.5th centile in European and ‘Other’ women. Utilising population haemoglobin A1c centiles adjusted by gestation may thus better guide management decisions.