Background: Screening for diabetes in early pregnancy is recommended for high-risk women, however, the optimal test for the diagnosis of early gestational diabetes mellitus (GDM) is unknown. Thus, the objective of this study was to evaluate hemoglobin A1c (HbA1c) as a diagnostic test for early GDM compared with two-step testing. Materials and Methods: Retrospective cohort of women with prior GDM or obesity who had HbA1c and two-step testing <21 weeks' gestation. Early GDM was diagnosed by 1 hour, 50 g oral glucose challenge test (GCT) ≥135 mg/dL and ≥2 abnormal values on 3 hour, 100 g oral glucose tolerance test or GCT >200 mg/dL. The area under the receiver operating characteristic curve (AUC) evaluated HbA1c for diagnosis of early GDM. Results: Of 243 women, 14 (5.8%) had early GDM by two-step testing. Median HbA1c levels were higher among women with GDM versus those without GDM (5.8% vs. 5.3%, p < 0.001). The AUC for HbA1c compared with two-step testing was 0.80 (95% CI 0.69-0.91). The optimal HbA1c threshold was 5.6% (64% sensitivity, 84% specificity). Conclusions: HbA1c is moderately predictive of early GDM compared with two-step testing, and a threshold lower than that used for diabetes diagnosis among nonpregnant adults is justified.