In this study, researchers evaluated markers that may be associated with undiagnosed type 2 diabetes (T2D) among women with early‐onset gestational diabetes mellitus (GDM). Women diagnosed with GDM prior to their 24th week of pregnancy were retrospectively analyzed. Outcomes from the 50-g glucose challenge test and 100-g oral glucose tolerance test, as well as the first available glycosylated hemoglobin (HbA1c), were evaluated as T2D markers in women diagnosed six to 12 weeks post-pregnancy. Researchers calculated the area under the curve (AUC) of the receiving operating characteristic (ROC) curve for each marker. A total of 69,417 pregnancies were identified during the study period, of which 3,321 (5%) were with GDM. Of the 261 pregnancies (8%) diagnosed before the 24th week, data were available for 219 (84%). Of the pregnancies with available data, 11 (5%) were diagnosed with T2D after pregnancy. The most significant indicator of undiagnosed T2D was elevated hemoglobin A1c (HbA1c; AUC 91% with 95% CI [81%‐100%]); in multivariate analysis, elevated HbA1c was the only independent marker for undiagnosed T2D (odds ratio [OR] 19; 95% CI [3.2‐109], C statistic = 88%). Using the ROC curve, the cutoff of T2D prediction was 5.8%, with 89% sensitivity, 86% specificity, 99.4% negative predictive value, and 23% positive predictive value. The authors recommended that women with early-onset GDM with HbA1c ≥5.8% should be treated as T2D patients, with a confirmed diagnosis made post-pregnancy.