OBJECTIVE: To study the association between peri-conceptional A1C and serious adverse pregnancy outcome (congenital malformations and perinatal mortality). RESEARCH DESIGN AND METHODS: Prospective data were collected in 933 singleton pregnancies complicated by type 1 diabetes. RESULTS: The risk of serious adverse outcome at different A1C levels was compared with the background population. The risk was significantly higher when peri-conceptional A1C exceeded 6.9%, and the risk tended to increase gradually with increasing A1C. Women with A1C exceeding 10.4% had a very high risk of 16%. Congenital malformation rate increased significantly at A1C above 10.4%, whereas perinatal mortality was increased even at A1C below 6.9%. CONCLUSIONS: These results support recent guidelines of preconceptional A1C levels <7% in women with type 1 diabetes.