This article provides the reader with relevant information regarding the association between level of glycemia and perinatal outcome in preexisting diabetes. Although the glycemic profile is a continuum in nature, different thresholds of glucose are associated with fetal complications such as stillbirth, spontaneous abortion, congenital anomalies, fetal macrosomia, and metabolic and respiratory complications. For each complication, a different targeted threshold of normality is required. Thus, although it is not always possible to achieve optimal glycemic control in all patients, any improvement will be beneficial because it will reduce the rate of complications for a given glucose threshold.