In pregnant women with preexisting type 1 diabetes mellitus, maintaining near-normal blood glucose levels decreases the rate of major congenital anomalies (defined as those causing death or a serious handicap necessitating surgical correction or medical treatment). Prolonged preconception control of blood sugar to near normal levels reduces the rate of major congenital anomalies close to those seen in women without diabetes (strength of recommendation [SOR]: A, based on prospective cohort studies and randomized controlled trial [RCT]). Intensive management reduces the risk of congenital anomalies more than conventional therapy, and lowers the risk of neonatal hypoglycemia (SOR: B, based on RCT). Very tight control does not reduce clinically significant neonatal morbidity but does increase the risk of maternal hypoglycemia (SOR: B, based on a systematic review)