AIMS: Young-onset type 2 diabetes mellitus (YT2DM) is an aggressive phenotype, with some claiming that diagnosis at <40 years of age represents a distinct higher risk group. Others have suggested restricting YT2DM to those diagnosed at <30 years of age. In this context, we examined whether pregnancy outcomes differ between women diagnosed with YT2DM at <30 years of age (T2D30) versus those diagnosed at 30 to <40 years of age (T2D40). METHODS: This retrospective analysis (2010-2019) compared pregnancy outcomes in women with pre-gestational T2D30 versus T2D40. Co-primary outcomes included preterm delivery, large for gestational age (LGA) infants, and pre-eclampsia. RESULTS: Compared to T2D40 (N = 69), T2D30 (N = 66) were significantly younger, had a longer duration of diabetes, and had higher rates of smoking (p <0.05 for all). In both groups, obesity affected ≥60% of women and similar rates of preterm delivery and LGA infants were seen. Women with T2D30 had at least a twofold increase in the adjusted odds of excess gestational weight gain (GWG). Rates of proteinuria and pre-eclampsia were increased in T2D30, although significance was lost following adjustment for factors such as glycaemia. Younger age of YT2DM diagnosis and longer duration of YT2DM (as continuous variables), but not maternal age, were independently associated with higher mean pregnancy HbA1c and excess GWG (p <0.05 for both). CONCLUSIONS: An earlier age of YT2DM diagnosis and/or longer duration of YT2DM were associated with excess GWG and a higher mean-pregnancy HbA1c. Rates of pre-eclampsia and proteinuria were increased in T2D30, likely mediated at least in part by factors such as glycaemia. Our findings suggest that the age of YT2DM diagnosis and/or duration of YT2DM, not just maternal age, should be considered when assessing pregnancy risks.