The previous Australasian Diabetes in Pregnancy Society (ADIPS) guidelines for the diagnosis of gestational diabetes mellitus (GDM) were formulated in 1991. These guidelines were revised in 2014 in accordance with the World Health Organization (WHO) recommendations. It was speculated that the new diagnostic criteria would increase the prevalence of GDM in Australia, but it is unclear whether pregnancy outcomes would also be different. Liverpool Hospital (in south-western Sydney) adopted the new WHO diagnostic criteria in February 2016. The aim of this study was to assess the difference in baseline characteristics and outcomes of women with GDM diagnosed with the new and old criteria. We present a retrospective study of women with GDM seen at our clinic between 2014 and 2016. There were 1029 women diagnosed with GDM using the old criteria (2014-2016) and 653 women diagnosed using the new criteria (2016-2017). Between the two groups there was no difference in ethnicity, previous GDM and gestational week when GDM was diagnosed. There was significant difference in the need for insulin therapy with a higher rate of insulin use (both basal as well as bolus) in women diagnosed with the old criteria (basal: 31.9 vs 24.8%, p=0.002; bolus: 32.9 vs 19.4%, p<0.001). As for pregnancy outcomes, there was no difference in mode of delivery, macrosomia, pre-maturity or peri-natal death. However, neonatal hypoglycaemia was more commonly observed with the old criteria (15.7% vs 5.5%). In conclusion, it appears that the significant differences between the two groups are related to treatment options and the occurrence of neonatal hypoglycaemia. This is likely related to the increased number of women being diagnosed with GDM at a lower threshold. The lower rate of insulin requirement among women diagnosed with the new criteria suggested that many of these women may have milder degree of maternal dysglycaemia.