Background: Evidence on risk factors, incidence and impact of SARS-CoV-2 infection in pregnant mothers and their babies has rapidly expanded but there is a lack of population level data to inform accurate incidence rates and unbiased descriptions of characteristics and outcomes. The primary aim of this study was to describe the incidence, characteristics and outcomes of hospitalized pregnant women with symptomatic and asymptomatic SARS-CoV-2 in the UK compared to pregnant women without SARS-CoV-2 in order to inform future clinical guidance and management. Methods and Findings: We conducted a national, prospective cohort study of all hospitalized pregnant women with confirmed SARS-CoV-2 from 1st March 2020 to 31st August 2020 using the UK Obstetric Surveillance System (UKOSS) across all 194 hospitals in the UK with a consultant-led maternity unit. Incidence was estimated using the latest national maternity data. Overall, 1148 hospitalized women had confirmed SARS-CoV-2 in pregnancy, 63% of which were symptomatic. Therefore, the estimated incidence of hospitalization with symptomatic SARS-CoV-2 was 2.0 per 1000 maternities (95% CI 1.9-2.2) and for asymptomatic SARS-CoV-2 was 1.2 per 1000 maternities (95% CI 1.1-1.4). Compared to pregnant women without SARS-CoV-2, women hospitalized with symptomatic SARS-CoV-2 were more likely to be overweight or obese (adjusted OR 1.86, 95% CI 1.39-2.48 and aOR 2.07, 95% CI 1.53-2.29 respectively), to be of Black, Asian or Other minority ethnic group (aOR 6.24, 95% CI 3.93-9.90, aOR 4.36, 95% CI 3.19-5.95 and aOR 12.95, 95% CI 4.93-34.01 respectively), and to have a relevant medical comorbidity (aOR 1.83, 95% CI 1.32-2.54). Compared to pregnant women without SARS-CoV-2, hospitalized pregnant women with symptomatic SARS-CoV-2 were more likely to be admitted to intensive care (aOR 57.67, 95% CI 7.80-426.70) but the absolute risk of poor outcomes was low. Cesarean births and neonatal unit admission were increased regardless of symptom status (symptomatic aOR 2.60, 95% CI 1.97-3.42 and aOR 3.08, 95% CI 1.99-4.77 respectively; asymptomatic aOR 2.02, 95% CI 1.52-2.70 and aOR 1.84, 95% 1.12-3.03 respectively). Iatrogenic preterm births were more common in women with symptomatic SARS-CoV-2 (aOR 11.43, 95% CI 5.07-25.75). The risks of stillbirth or neonatal death were not significantly increased, regardless of symptom status but numbers were small. The limitations of this study include the restriction to women hospitalized with SARS-CoV-2, who may by nature of their admission have been at greater risk of adverse outcome. Conclusions: We have identified factors that increase the risk of symptomatic and asymptomatic SARS-CoV-2 in pregnancy. The increased risks of cesarean and iatrogenic preterm birth provide clear evidence of the indirect impact of SARS-CoV-2 on mothers and maternity care in high income settings. Clinicians can be reassured that the majority of women do not experience severe complications of SARS-CoV-2 in pregnancy and women with mild disease can be discharged to continue their pregnancy safely.