Objective. To investigate the relationship between the level of C-peptide, the severity of arterial stiffness and the presence of obesity in preeclampsia and gestational diabetes. Materials and methods. The study comprised 114 pregnant women at more than 24 weeks' gestation, including patients with gestational diabetes and preeclampsia (n = 22), preeclampsia (n = 23), gestational diabetes (n = 36), and control subjects (n = 33). The patient evaluation included fasting serum level of C-peptide, augmentation index, and body mass index. The statistical analysis involved correlation coefficients, analysis of variance, and empirical ROC curve. Results. There was an independent association between C-peptide level and preeclampsia (p = 0.04) and obesity (p <0.001), but not concurrent gestational diabetes and preeclampsia. The augmentation index threshold was determined at -45% in relation to the presence of preeclampsia among patients with a C-peptide level above 3.1 ng/ml (sensitivity 0.92; specificity 0.59; OR = 17.14; 95%CI = [1.80; 163.8]). No statistically significant correlation was found between augmentation index and body mass index (r = -0.01; p = 0.9). Conclusion. The study established the values of C-peptide and augmentation index that may be suggested for further evaluation as possible predictors for preeclampsia and cardiovascular diseases after preeclampsia, regardless of the presence of gestational diabetes.