Maternal Haemoglobin A1C Level as a Predictor of Adverse Pregnancy Outcomes in Women with early versus late Gestational Diabetes Mellitus

Background The objective was to evaluate the association between baseline maternal haemoglobin A1C (HbA1C) levels and adverse pregnancy outcomes in early (< 24 weeks) and late (≥ 24 weeks) gestational diabetes mellitus (GDM). Study Design This was a retrospective cohort undertaken at a public hospital in Hong Kong between 2021 and 2024. The associations between baseline HbA1C level (taken within 1–2 weeks after the diagnosis of GDM) and pregnancy outcomes were analysed using multivariate logistic regression. Results Of 1,531 women studied, 554 and 977 had early and late onset GDM, respectively. In late onset GDM, baseline HbA1C ≥ 6.0% showed associations with birth weight > 97th percentile (adjusted odds ratio [95% confidence interval = 8.2 [2.2–31.2]), large for gestational age (LGA) foetus (3.0 [1.3–6.7]), neonatal hypoglycaemia (5.7 [2.4–13.2]), hypertensive disorder (5.5 [1.7–17.8]), and insulin treatment (190.3 [21.1–1717.4]). The risk of low birth weight (< 2.5kg) followed a U-shaped distribution, with the lowest risk in the HbA1C 5.5–5.9% group (0.34 [0.15–0.78]). However, in early onset GDM, the associations were limited to neonatal hypoglycaemia (3.8 [1.5–10.0]) and insulin treatment (39.7 [7.8–200.9]). Conclusion The significance of baseline HbA1C is different between early and late-onset GDM. In contrast to baseline HbA1C levels ≥ 6.0% at late-onset GDM, that at early-onset GDM did not identify LGA and hypertensive disorders. Besides, high HbA1C levels as well as low HbA1C level (< 5%) at late onset GDM are associated with increased risk of LBW.