The association between maternal diabetes mellitus and congenital malformations in newborns may well be causal; however, the teratogenic mechanism remains unclear. 1 The prevalence and the described type of malformations vary among different studies, and a predictable malformation syndrome has not been identified. 1 This chapter discusses the prevalence and types of structural congenital malformations in infants of mothers with diabetes mellitus (IDM) or gestational diabetes mellitus (IGDM). Their etiology and pathogenesis are presented in two other chapters of this book.