Background
There is debate as to the most appropriate diagnostic criteria to diagnose gestational diabetes mellitus (GDM). The proposed International Association of Diabetes and Pregnancy Study Groups (IADPSG) criteria have recently been endorsed by various bodies, but there remains no national consensus.

Aim
To assess the perinatal outcomes of women with GDM classified according to the 1998 Australasian Diabetes in Pregnancy Society (ADIPS) criteria compared to those with GDM by the IADPSG criteria.

Materials and Methods
Results of glucose tolerance tests performed between January 2011 and January 2014 were linked with the perinatal data of those who delivered singleton fetuses after 24 weeks’ gestation. Analysed data included birthweight, gestational age at birth, macrosomia, mode of delivery, perinatal mortality, nursery admission, maternal body mass index, and gestational weight gain.

Results
Of 3571 women, 466 (13%) and 559 (16%) met the criteria for the 1998 ADIPS and IADPSG criteria for GDM, respectively. Those with GDM according to the IADPSG criteria only (6%) were more obese (95% CI 2.3–4.8 kg/m2), delivered neonates on average 106 g heavier (95% CI 19–193 g) and had more fetal macrosomia (18% vs 11%, P = 0.002) than those with normal glucose tolerance.

Conclusions
The IADPSG criteria for GDM identified a group of women at previously unrecognised increased risk of adverse perinatal outcomes. Adopting the IADPSG criteria would increase the number of women diagnosed with GDM by 20%; however, the improvements in perinatal morbidity, in addition to potential long‐term benefits, may justify the increase in healthcare workload.