The present multicenter retrospective cohort study was performed by the Spanish Diabetes and Pregnancy Study Group at nine tertiary university hospitals in Spain. Medical records were reviewed to identify women with pregestational DM (type 1 or type 2) who gave birth to twins between January 1, 2005, and December 31, 2010. Diabetes was defined according to the classification of the American Association of Diabetes [8]. For each eligible woman with a twin pregnancy, the next consecutive woman with pregestational DM of the same type and a singleton pregnancy was included in a control group. The study was approved by the institutional review board at the University Hospital of the Canary Islands. Informed consent was not needed because of the retrospective nature of the study.