Baseline HbA1c to Identify High-Risk Gestational Diabetes: Utility in Early vs Standard Gestational Diabetes

Context The increasing prevalence of gestational diabetes mellitus (GDM) necessitates risk stratification directing limited antenatal resources to those at greatest risk. Recent evidence demonstrates that an early pregnancy glycated hemoglobin (HbA1c ≥5.9% (41 mmol/mol) predicts adverse pregnancy outcomes. Objective To determine the optimal HbA1c threshold for adverse pregnancy outcomes in GDM in a treated multiethnic cohort and whether this differs in women diagnosed <24 vs ≥24 weeks' gestation (early vs standard GDM). Design and Setting This was a retrospective cohort study undertaken at the Royal Prince Alfred Hospital Diabetes Antenatal Clinic, Australia, between 1991 and 2011. Patients and Interventions Pregnant women (N = 3098) underwent an HbA1c (single-laboratory) measurement at the time of GDM diagnosis. Maternal clinical and pregnancy outcome data were collected prospectively. Main Outcome Measure The association between baseline HbA1c and adverse pregnancy outcomes in early vs standard GDM. Results HbA1c was measured at a median of 17.6 ± 3.3 weeks' gestation in early GDM (n = 844) and 29.4 ± 2.6 weeks' gestation in standard GDM (n = 2254). In standard GDM, HbA1c >5.9% (41 mmol/mol) was associated with the greatest risk of large-for-gestational-age (odds ratio [95% confidence interval] = 2.7 [1.5-4.9]), macrosomia (3.5 [1.4-8.6]), cesarean section (3.6 [2.1-6.2]), and hypertensive disorders (2.6 [1.1-5.8]). In early GDM, similar HbA1c associations were seen; however, lower HbA1c correlated with the greatest risk of small-for-gestational-age (P trend = 0.004) and prevalence of neonatal hypoglycemia. Conclusions Baseline HbA1c >5.9% (41 mmol/mol) identifies an increased risk of large-for-gestational-age, macrosomia, cesarean section, and hypertensive disorders in standard GDM. Although similar associations are seen in early GDM, higher HbA1c levels do not adequately capture risk-limiting utility as a triage tool in this cohort.