Care of the diabetic pregnant woman requires a proper understanding of anticipated changes in insulin therapy as a guide for establishing and maintaining strict glucose control. Changes in daily insulin doses were reviewed for 58 pregnancies of 50 insulin-dependent women who were followed for 26 +/- 3 weeks (mean +/- 1 SD) before delivery. By late gestation, insulin was administered on two or three occasions each day using a combination of regular--and intermediate--acting preparations in 55 (95%) pregnancies. Regardless of the metabolic control and duration of diabetes, averaged daily insulin requirements increased twofold from earlier in pregnancy. Following initial hospitalization, insulin requirements often decreased before increasing almost linearly between 2 and 9 months gestation. Fluctuations in insulin requirements were greatest during the last trimester. Insulin demand dropped precipitously after delivery and was two-thirds the averaged prepregnancy insulin dose or one-third the dose at 9 months gestation by the third postpartum day. The total average insulin dose was the same as that before pregnancy by the end of the first postpartum week. Explanations for these changes in prescribing insulin are described.