INS and diabetes mellitus: The aims of this study were to: (1) estimate the prevalence of GDM in pregnant women in 2012 in France; (2) assess perinatal outcomes after 22 weeks of pregnancy in the presence of GDM compared with pregestational diabetes or no diabetes; (3) determine whether perinatal outcomes differed according to whether GDM was treated by insulin or diet alone; and (4) increase the validity of conclusions by controlling for immortal time bias and contamination by data from women with undiagnosed pregestational diabetes.