OBJECTIVE: To assess the safety and efficacy of insulin aspart (IAsp) versus regular human insulin (HI) in basal-bolus therapy with NPH insulin in pregnant women with type 1 diabetes. RESEARCH DESIGN AND METHODS: Subjects (n = 322) who were pregnant or planning pregnancy were randomized to IAsp or HI as meal-time insulin in an open-label, parallel-group, multicenter study. Subjects had A1C < or =8% at confirmation of pregnancy. Insulin doses were titrated toward predefined glucose targets and A1C <6.5%. Outcomes assessed included risk of major maternal hypoglycemia, A1C, plasma glucose profiles, and maternal safety outcomes. RESULTS: Major hypoglycemia occurred at a rate of 1.4 vs. 2.1 episodes/year exposure with IAsp and HI, respectively (relative risk 0.720 [95% CI 0.36-1.46]). Risk of major/major nocturnal hypoglycemia was 52% (RR 0.48 [0.20-1.143]; P = NS) lower with IAsp compared with HI. A1C was comparable with human insulin in second (IAsp-HI -0.04 [-0.18 to 0.11]) and third (-0.08 [-0.23 to 0.06]) trimesters. A total of 80% of subjects achieved an A1C < or =6.5%. At the end of first and third trimesters, average postprandial plasma glucose increments were significantly lower with IAsp than HI (P = 0.003 and P = 0.044, respectively), as were mean plasma glucose levels 90 min after breakfast (P = 0.044 and P = 0.001, respectively). Maternal safety profiles and pregnancy outcomes were similar between treatments. CONCLUSIONS: IAsp is at least as safe and effective as HI when used in basal-bolus therapy with NPH insulin in pregnant women with type 1 diabetes and may potentially offer some benefits in terms of postprandial glucose control and preventing severe hypoglycemia.