Background
The adoption of the International Association of Diabetes Study Groups (IADPSG) criteria for gestational diabetes mellitus (GDM) in Australia has been controversial. Obesity in pregnancy is also a growing concern.

Aims
To assess the impact of IADPSG criteria on the incidence of GDM and pregnancy outcomes, and to compare this to the effect of obesity, particularly among women who would not have GDM by the Australasian Diabetes in Pregnancy Society 1998 criteria (ADIPS1998).

Material and Methods
A retrospective observational cohort study linking results of 75 g glucose tolerance tests with demographic and pregnancy data was conducted.

Results
In our cohort of 6175 pregnancies, GDM was present in 926 (15%) women by the ADIPS1998 criteria; it increased to 1098 (17.8%) women by the IADPSG criteria. Among the 5248 pregnancies which did not meet the ADIPS1998 criteria and were not treated for GDM, women with IADPSG GDM had increased risk of gestational hypertension, pre‐eclampsia, induction of labour (IOL), primary caesarean section (PCS) and large for gestational age (LGA) compared to women without GDM (all P < 0.05), whereas obese women had increased risk of gestational hypertension, pre‐eclampsia, IOL, PCS, small for gestational age (SGA) and shoulder dystocia compared to women of normal weight (all P < 0.05). On multivariate analysis, IADPSG GDM was an independent risk factor only for IOL (P = 0.04) and LGA (<0.001). Obesity was an independent risk factor for gestational hypertension, pre‐eclampsia, IOL, PCS, shoulder dystocia and SGA (all P < 0.001). Conclusions Within our population, of women who are not currently treated for GDM, obesity is associated with greater pregnancy risk than GDM diagnosed by IADPSG criteria.