Aims: To investigate the relationship between insulin resistance (IR) and subsequent gestational hypertension (GH) or preeclampsia (PE) in normoglycemic and in gestational diabetic pregnant women. Furthermore, we tested whether this association was independent of the prepregnancy body mass index (BMI). Methods: Each participant underwent a 75-gram oral glucose tolerance test (OGTT) according to World Health Organization recommendation criteria with determination of serum glucose and C-peptide concentrations. IR was determined as a C-peptide-to-glucose ratio at the fasting (FCGR) state and at 2 h (2CGR) after load. Results: A total of 2,954 women were included with a singleton pregnancy, delivery at term, no chronic hypertension, and data on both glucose and C-peptide. Of these women, 183 (6.2%) developed GH and 49 (1.7%) PE. Gestational diabetes mellitus (GDM) was diagnosed in 6.0% of the participants. The FCGR and 2CGR were significantly higher in all women with GH, irrespective of their BMI, compared to the normotensive group; however, the PE and the normotensive groups had similar FCGR and 2CGR values. Conclusions: The present study suggests that IR at the OGTT is associated with the later development of GH; on the other hand, it is not associated with PE. These relationships are independent of the maternal BMI.