AIMS: To compare perinatal outcomes and postpartum glucose tolerance between women diagnosed with gestational diabetes mellitus (GDM) before 20 weeks of gestation (EGDM) and those diagnosed at or after 24 weeks of gestation (LGDM) in a Japanese population. MATERIALS AND METHODS: Data were obtained from a prospective GDM registry. Multivariate analysis was conducted to examine the association between the timing of GDM diagnosis (EGDM vs LGDM) and perinatal outcomes (preterm birth, small for gestational age, large for gestational age, pregnancy-induced hypertension, and neonatal hypoglycemia), as well as postpartum glucose intolerance. RESULTS: A total of 1,275 mother-infant pairs were analyzed for perinatal outcomes. Of these, 924 women underwent postpartum testing for glucose intolerance. No significant differences in perinatal outcomes were observed between the EGDM and LGDM groups, except that overweight/obese women with EGDM had 2.5-fold higher rate of preterm birth than those with LGDM. Postpartum glucose intolerance was 1.5 times more likely in the EGDM group than in the LGDM group. CONCLUSIONS: Women with EGDM had a significantly higher risk of postpartum glucose intolerance than those with LGDM, despite similar perinatal outcomes between the two groups.