Objective- Insulin sensitivity and secretion during early and late pregnancy were assessed in women with normal glucose tolerance and gestational diabetes (GDM).

Research design and Methods- Oral glucose tolerance test (OGTT) was performed in 903 women at 16–20th gestational week, of whom 37 had GDM (GDM1 group) and 859 repeated the OGTT at week 26–30. At the second test 55 became GDM (GDM2 group), the others remained normotolerant (ND group). Insulin sensitivity (QUICKI and OGIS from the OGTT) and beta cell function (as the ratio of the areas under the insulin and glucose concentration curves, adjusted for insulin sensitivity) were assessed in both tests.

Results- In early pregnancy QUICKI was not different in the three groups, while OGIS was lowest in GDM2, intermediate in GDM1 and highest in ND. In late pregnancy both indices were reduced in GDM compared to ND and lower than in early pregnancy. In early pregnancy GDM1 but not GDM2 had lower beta cell function than ND. During late visit, GDM2 also showed impaired beta cell function compared to ND; furthermore, the adaptation to the increase to insulin resistance from early to late pregnancy was defective in GDM2.

Conclusions- In early pregnancy insulin sensitivity as assessed from the OGTT but not from fasting measurements is impaired in women who become GDM. Beta cell function impairment is evident only when GDM is manifest and is characterized by inappropriate adaptation to the pregnancy-induced increase in insulin resistance.