Introduction: We hypothesized that the pathophysiology of glucose intolerance might differ when comparing women in earlier pregnancy (before 20 weeks of gestational age) and those at mid-pregnancy (26–30 weeks of gestational age). We used the homeostasis model assessment of β-cell function (HOMA-β) and HOMA-IR as indicators of insulin secretion and insulin resistance, respectively. Methods: This retrospective study looked at insulin secretion and resistance in women with normal glucose tolerance (NGT) or gestational diabetes (GDM) diagnosed by 75g oral glucose tolerance tests using criteria of the Japan Society of Obstetrics and Gynecology during both earlier pregnancy (NGT: n=170, GDM: n=62) and mid-pregnancy (NGT: n=204, GDM: n=124) using HOMA-β and HOMA-IR. Results: There was no difference in HOMA-β when comparing the NGT and GDM groups in earlier pregnancy (120 ± 85% vs. 108 ± 53%). The HOMA-IR was significantly higher in the GDM group than in the NGT group (1.9 ± 1.1 vs. 1.3 ± 0.6). The NGT and GDM groups had similar HOMA-β levels at 26–30 weeks, but the HOMA-IR was again higher in the GDM group compared to the NGT group. Conclusions: Insulin secretion levels were maintained and levels of insulin resistance were elevated in GDM diagnosed during both earlier and later gestational ages. GDM diagnosed earlier in the pregnancy did not show different characteristics using HOMA-β and HOMA-IR.