The objective of the study was to assess the temporal changes in vascular function during pregnancy in healthy women and in those with gestational diabetes mellitus (GDM). Assessment of vascular function was done at three time points, 11–13+6 weeks+days, 20–22+6 weeks+days and 30–32+6 weeks+days, by flow-mediated dilatation (FMD), augmentation index (AIx) and carotid-radial pulse wave velocity (crPWV) in women (n=100) with singleton pregnancies. Out of the 100 women, 20 developed GDM, who were compared with 20 healthy, age-matched pregnant women in a nested case-control design. Women with GDM had lower FMD% in the third compared to the first trimester (6.77 (4.36–9.96) vs 9.76 (6.66–16.61)%; p = 0.026); however, FMD% was similar on inter-group comparison between GDM and healthy pregnancies. AIx was significantly higher in GDM than healthy pregnancies at both first (15.35 ± 10.57 vs 6.45 ± 9.81%; p<0.05) and second trimesters (15.00 ± 8.44 vs 2.50 ± 9.01%; p<0.05). A higher AIx in early pregnancy differentiates women with GDM from those with healthy pregnancies.