The first few weeks after conception are a critical period for the embryo of a diabetic mother. If the mother is in good glycaemic control during this time, the risk of spontaneous abortion or major congenital malformation is low. Otherwise, the risk increases in proportion to the degree of blood glucose elevation. Glycohaemoglobin determination in the first trimester can be useful in retrospectively evaluating the degree of glycaemic control present around the time of conception and in roughly estimating the risk of spontaneous abortion or major malformation. For women with high risk, early prenatal diagnosis of congenital anomalies is warranted using detailed ultrasound examination, fetal echocardiography and alpha-fetoprotein determinations. Encouraging diabetic women to achieve strict control prior to conception should virtually eliminate the excess risk of spontaneous abortion or major malformation.