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IMPORTANCE: Hyperglycemia-related pregnancy complications occur in 50% of pregnant women with type 1 diabetes. Closed-loop insulin systems improve glycemia outside of pregnancy but have had limited testing in pregnancy. OBJECTIVE: To assess the efficacy of a closed-loop system in pregnancy. DESIGN, SETTING, AND PARTICIPANTS: Open-label trial enrolling pregnant women with type 1 diabetes at 14 clinical centers in Canada and Australia before 14 weeks' gestation with follow-up until 6 weeks postpartum. Enrollment occurred between June 2021 and July 2024 and follow-up was completed in March 2025. INTERVENTIONS: Participants were randomized 1:1 to closed-loop therapy (n = 46) or standard care (insulin pump or multiple daily insulin injections) (n = 45) with continuous glucose monitoring. MAIN OUTCOMES AND MEASURES: The primary outcome was the percentage of time spent in the pregnancy-specific glucose range (63-140 mg/dL), measured by continuous glucose monitoring from 16 to 34 weeks' gestation. RESULTS: Among 94 enrolled participants, 3 experienced pregnancy loss prior to randomization, 91 were randomized (mean age, 31.7 [SD, 5.2] years; early pregnancy hemoglobin A1c, 7.4% [SD, 1.0%]), and 88 were included in the primary analysis. The mean percentage of time spent in the pregnancy-specific glucose range from 16 to 34 weeks' gestation was 65.4% in the closed-loop group and 50.3% in the standard care group (mean adjusted difference, 12.5 [95% CI, 9.5-15.6] percentage points; P < .001). There was 1 episode of severe hypoglycemia in the closed-loop group, and there were 2 episodes of diabetic ketoacidosis in the closed-loop group and 1 in the standard care group. CONCLUSION AND RELEVANCE: Pregnant women with type 1 diabetes using a closed-loop system spent significantly more time in the pregnancy-specific glucose range than those receiving standard care. These findings support the use of this closed-loop system in pregnant women with type 1 diabetes. TRIAL REGISTRATION: ClinicalTrials.gov Identifier: NCT04902378.