BACKGROUND: Maternal diabetes increases risk for congenital malformations, particularly cardiac outflow tract defects. Maternal diabetes inhibits expression of Pax3 in neuroepithelium through hyperglycemia-induced oxidative stress. The neuroepithelium gives rise to the neural crest, and Pax3 expression in cardiac neural crest (CNC) is required for CNC migration to the heart and for outflow tract septation. Here we tested whether maternal diabetes, through hyperglycemia-induced oxidative stress, before the onset of CNC delamination, impairs CNC migration and cardiac outflow tract septation. METHODS: CNC migration was mapped in mouse embryos whose mothers were diabetic, or transiently hyperglycemic, or in which oxidative stress was transiently induced, using reporters linked to Pax3 expression. CNC apoptosis was examined by TUNEL assay. Outflow tract septation was examined histologically and by gross inspection. RESULTS: Few, if any, migrating CNC cells were observed in embryos of diabetic mice, and this was associated with increased apoptosis along the path of CNC migration. Outflow tract defects were significantly increased in fetuses of diabetic mice. Notably, induction of hyperglycemia or oxidative stress on the day prior to the onset of Pax3 expression and CNC migration also impaired CNC migration, increased apoptosis, and caused outflow tract defects. However, antioxidants administered on the day prior to the onset of Pax3 expression and CNC migration prevented these effects of hyperglycemia or oxidative stress. CONCLUSIONS: In diabetic pregnancy, oxidative stress, which inhibits expression of genes required for CNC viability, causes subsequent CNC depletion by apoptosis during migration, which leads to outflow tract defects. Birth Defects Research (Part A), 2008. © 2008 Wiley-Liss, Inc.