New diagnostic criteria for gestational diabetes mellitus (GDM) have recently been published. We wished to evaluate what impact these new criteria would have on GDM prevalence and outcomes in a predominantly European population.The Atlantic Diabetes In Pregnancy (DIP) programme performed screening for GDM in 5,500 women with an oral glucose tolerance test at 24-28 weeks. GDM was defined according to the new International Association of Diabetes and Pregnancy Study Groups (IADPSG) criteria and compared with previous WHO criteria; maternal and neonatal adverse outcomes were prospectively recorded.Of the participants, 12.4% and 9.4% were diagnosed with GDM using IADPSG and WHO criteria, respectively. IADPSG GDM pregnancies were associated with a statistically significant increased incidence of adverse maternal outcomes (gestational hypertension, polyhydramnios and Caesarean section) and neonatal outcomes (prematurity, large for gestational age, neonatal unit admission, neonatal hypoglycaemia and respiratory distress). The odds ratio for the development of these adverse outcomes remained significant after adjustment for maternal age, body mass index and non-European ethnicity. Those women who were classified as having normal glucose tolerance by WHO criteria but as having GDM by IADPSG criteria also had significant adverse pregnancy outcomes.GDM prevalence is higher when using newer IADPSG, compared with WHO, criteria, and these women and their offspring experience significant adverse pregnancy outcomes. Higher rates of GDM pose a challenge to healthcare systems, but improved screening provides an opportunity to attempt to reduce the associated morbidity for mother and child.