['To monitor beta-cell function and insulin sensitivity longitudinally in a large cohort of pregnant women to elucidate mechanisms that influence glycemic control in pregnancy.', 'Five hundred and fifty-three pregnant Scandinavian women underwent 75 g oral glucose tolerance test (OGTT) at weeks 14-16 and 30-32. Insulin sensitivity (Matsuda index) and beta-cell function (ratio of AUC(insulin) to AUC(glucose), AUC(ins/glc)) were calculated from 520 complete tests, and subsequently beta-cell function was adjusted for insulin sensitivity, rendering an oral disposition index (DI(o)).', 'Eleven women (2.1%) had gestational diabetes mellitus (GDM1) at weeks 14-16, and 49 (9.4%) at weeks 30-32 (GDM2), which is higher than that previously reported in this region. In the subdivision of OGTT, more overweight (body mass index>25) was found in glucose-intolerant groups (glucose-tolerant women (normal glucose tolerance, NGT) 38 versus GDM2 women 58 and GDM1 women 82%, P<0.005). In early pregnancy, insulin sensitivity was lowest in GDM1, intermediate in GDM2, and highest in NGT. In late pregnancy, insulin sensitivity decreased in all groups, most in gestational diabetes. beta-cell function demonstrated minor shifts during pregnancy, but when adjusted for decreasing insulin sensitivity, DI(o) levels fell by 40% (P<0.001). DI(o) was significantly attenuated relative to glucose intolerance (GDM1 25% and GDM2 53%) during pregnancy. In overweight women, DI(o) levels were lower throughout pregnancy (P<0.001 versus normal weight women), this reduction was significant (P<0.01) in both NGT (21-25%) and GDM2 subjects (26-49%).'] Is overweight associated with impaired beta-cell function during pregnancy : a longitudinal study of 553 normal pregnancies?