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Objective: To investigate the effect of restriction measures implemented to mitigate severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission during the coronavirus disease 2019 (COVID-19) pandemic on pregnancy duration and outcome., Methods: A before-and-after study was conducted with cohort sampling in three maternity hospitals in Melbourne, Australia, including women who were pregnant when restriction measures were in place during the COVID-19 pandemic (estimated conception date between 1 November 2019 and 29 February 2020) and women who were pregnant before the restrictions (estimated conception date between 1 November 2018 and 28 February 2019). The primary outcome was delivery before 34 weeks' gestation or stillbirth. The main secondary outcome was a composite of adverse perinatal outcomes. Pregnancy outcomes were compared between women exposed to restriction measures and unexposed controls using the χ-square test and modified Poisson regression models, and duration of pregnancy was compared between the groups using survival analysis., Results: In total, 3150 women who were exposed to restriction measures during pregnancy and 3175 unexposed controls were included. Preterm birth before 34 weeks or stillbirth occurred in 95 (3.0%) exposed pregnancies and in 130 (4.1%) controls (risk ratio (RR), 0.74 (95% CI, 0.57-0.96); P = 0.021). Preterm birth before 34 weeks occurred in 2.4% of women in the exposed group and in 3.4% of women in the control group (RR, 0.71 (95% CI, 0.53-0.95); P = 0.022), without evidence of an increase in the rate of stillbirth in the exposed group (0.7% vs 0.9%; RR, 0.83 (95% CI, 0.48-1.44); P = 0.515). Competing-risks regression analysis showed that the effect of the restriction measures on spontaneous preterm birth was stronger and started earlier (subdistribution hazard ratio (HR), 0.81 (95% CI, 0.64-1.03); P = 0.087) than the effect on medically indicated preterm birth (subdistribution HR, 0.89 (95% CI, 0.70-1.12); P = 0.305). The effect was stronger in women with a previous preterm birth (RR, 0.42 (95% CI, 0.21-0.82); P = 0.008) than in parous women without a previous preterm birth (RR, 0.93 (95% CI, 0.63-1.38); P = 0.714) (P for interaction = 0.044). Composite adverse perinatal outcome was less frequent in the exposed group than in controls (all women: 2.1% vs 2.9%; RR, 0.73 (95% CI, 0.54-0.99); P = 0.042); women with a previous preterm birth: 4.5% vs 8.4%; RR, 0.54 (95% CI, 0.25-1.18); P = 0.116)., Conclusions: Restriction measures implemented to mitigate SARS-CoV-2 transmission during the COVID-19 pandemic were associated with a reduced rate of preterm birth before 34 weeks. This reduction was mainly due to a lower rate of spontaneous prematurity. The effect was more substantial in women with a previous preterm birth and was not associated with an increased stillbirth rate. © 2021 International Society of Ultrasound in Obstetrics and Gynecology., (© 2021 International Society of Ultrasound in Obstetrics and Gynecology.)