Background
Gestational diabetes mellitus (GDM) is glucose intolerance first diagnosed during pregnancy not due to overt diabetes. Recent changes to the diagnostic guidelines have been shown to increase the apparent occurrence of GDM.

Aim
The aim of this study was to compare retrospectively the neonatal outcomes between groups defined using the new and old criteria to assess the impact of guideline changes on pregnancy outcomes.

Methods
The study was of singleton babies delivered of 641 women, who had oral glucose tolerance testing and pregnancy care at a single tertiary centre between 2011 and 2015.

Results
Compared to the population of women not now considered to have GDM by International Association of Diabetes and Pregnancy Study Groups criteria (two‐hour glucose concentration ≤8.4 mmol/L), neonates born to women with the new lower fasting criterion (5.1–5.4 mmol/L) and/or the new 60‐min group (glucose ≥10 mmol/L) combined were significantly more likely to have birthweight ≥90th percentile (22% vs 5%, P < 0.0001). In contradistinction, there was a significant excess number of small‐for‐dates babies (birthweight ≤10th percentile) in all subgroups previously diagnosed and treated for GDM by the Australian Diabetes in Pregnancy Society criteria (17% vs 7%, P = 0.001). Rates for lower uterine segment caesarean section, admission to the neonatal intensive care unit / special care nursery and Apgar scores at one and five minutes were not statistically different across all groups. Conclusions Outcomes support the lowering of the fasting criterion to extend management of GDM to limit growth of large birthweight neonates. An unexpected outcome was that in women previously treated for GDM, there were increased numbers of low‐birthweight neonates.