Background
The diagnostic criteria for gestational diabetes (GDM) have been controversial following the International Association for Pregnancy Study Groups (IADPSG) 2010 recommendations to lower the diagnostic fasting blood sugar level threshold.

Aim
To assess the pregnancy‐related outcomes of women according to the different diagnostic criteria for GDM adjusting for body mass index categories.

Method
A retrospective observational cohort study was conducted on 4081 pregnant women with positive 50 g glucose challenge test but without pre‐gestational diabetes. Participants were grouped into four cohorts: no GDM (control group); GDM on Australasian Diabetes in Pregnancy (ADIPS) 1998 criteria only (treated); GDM on IADPSG 2010 criteria only (untreated); and GDM on both criteria (treated). The association of each cohort with pregnancy outcome measures, including birthweight centile, delivery gestation, primary caesarean section, shoulder dystocia and stillbirth, together with the effect of obesity, were examined.

Results
Women diagnosed with GDM according to the IADPSG 2010 (untreated) but not the ADIPS 1998 criteria (treated) had an increased risk of being large for gestational age (LGA) (odds ratio (OR) = 2.45, 95% CI: 1.46–4.12, P = 0.001) and primary caesarean section (OR = 2.03, 95% CI: 1.23–3.35, P = 0.006) compared to control women. Among the women in this untreated group and women without GDM, obese women had an increased risk of LGA (OR = 3.82, 95% CI: 2.87–5.10, P < 0.001), shoulder dystocia (OR = 1.50, 95% CI: 1.03–2.19, P = 0.04) and primary caesarean section (OR = 1.63, 95% CI: 1.26–2.10, P < 0.001), compared to those women of normal weight. These associations remained significant on multivariate analysis. Conclusion Untreated women who would be diagnosed with GDM using the new criteria have an increased risk of pregnancy complications, with maternal obesity having an even greater risk.