(Abstracted from Lancet 2017;390:2347–2359) It is established that optimal maternal glycemic control throughout pregnancy is associated with a decrease in adverse fetal outcomes. A proper glycemic index can be difficult to control in women with type 1 diabetes during pregnancy because of the complexity of insulin dose adjustment, gestational changes in insulin sensitivity, and day-to-day changes to insulin absorption in late pregnancy. Nationwide UK data show the rates of severe hypoglycemia, particularly in early pregnancy, are 5 times higher among pregnant women than nonpregnant and that only 40% of women with type 1 diabetes achieve target glycated hemoglobin (HbA1c) after 24 weeks of gestation. Continuous glucose monitoring (CGM) provides real-time quantitative information on the direction and rate of change of glucose levels.