BACKGROUND: in Australia, and globally, rates for gestational diabetes mellitus (GDM) have risen dramatically in recent decades. This is of concern as GDM is associated with adverse pregnancy outcomes and additional health-care costs. Factors linked to increasing incidence include older maternal age and non-Caucasian ethnicity. However, as yet, there is no clear consensus on the magnitude of effect associated with these factors in combination. This study therefore investigated the effect of maternal age and country/region of birth on GDM incidence. METHODS: all women who gave birth in Victoria, Australia in 2005 and 2006 (n=133,359) were included in this population-based cross-sectional study. Stratified cross-tabulations were conducted to examine the incidence of GDM by maternal age group and country/region of birth. Primiparous women were further analysed separately from parous women. The proportion of women with GDM was reported, along with the χ(2) for linear trend. FINDINGS: whilst women born outside Australia constituted just 24.6% of women giving birth during the study period, they accounted for 41.4% of GDM cases. The highest GDM incidence was seen among Asian women at 11.5%, compared with Australian born women at 3.7%. There was strong evidence that women born in all regions except North America were increasingly likely to develop GDM in pregnancies at older ages (p<0.001).On examining age related GDM trends by maternal region of birth, higher rates were seen across all regions studied but were most marked among women born in Asia and the Middle East. CONCLUSIONS: older maternal age and non-Australian birth increased a woman's risk of developing GDM and this increase was most evident among Asian women. As GDM is associated with adverse maternal and infant outcomes, it is important to explore ways of preventing GDM, and to put in place strategies to effectively manage GDM during pregnancy and to reduce the later risk of developing type 2 diabetes. Pregnancy presents midwives with a unique opportunity to provide education and to encourage dietary and behavioural modifications as women have repeated contact with the health system during this time.