Major congenital anomalies (MCAs) are defined as those which are lethal and/or which cause major handicaps and/or which require surgery. While the general population risk for having a baby with an MCA is one to three percent, that for a woman who has pregestational diabetes mellitus is three to six times as great. Fetal organogenesis takes place from the fifth to the eighth post-menstrual weeks. A body of evidence suggests that maternal hyperglycemia early in gestation is either contributory to fetal teratogenesis or is a surrogate marker for the causative agent of MCAs in infants of diabetic mothers. Given that organogenesis begins at the time of a woman's first recognition of pregnancy, and given the one or more weeks thereafter needed to establish prenatal care, it is essential that the woman who has diabetes attempt to lower her glucose concentrations and treat any intercurrent complications (e.g. hypertension, retinopathy) prior to the initiation of efforts to conceive. The elements of a preconception care program for pregestational diabetic women include health care provider education, patient recruitment and retention, education about family planning and distribution of contraception, the diagnosis and treatment of complications of diabetes, the establishment of control of maternal glucose, and the transition to intensive prenatal care. The support and encouragement of health care personnel, family, and friends is essential for patient retention and compliance with the often rigorous demands of good preconception care.