Objectives: To identify factors associated with adverse maternal outcomes during the coronavirus disease 2019 (COVID-19) pandemic. Methods: This was a single-centre prospective cohort study at a maternity department in a public general hospital in Rio de Janeiro. All pregnant women evaluated for emergency care, labour and delivery, respiratory symptoms, obstetric reasons or medical reasons between May 2020 and March 2022 at the study institution were invited to enrol in this study. The endpoint was maternal mortality or intensive care unit (ICU) admission. Results: In total, 1609 pregnant women were enrolled in this study. Of these, 25.5% (n=410) were infected with severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) based on reverse transcription polymerase chain reaction or an antigen test. There were 21 deaths and 67 ICU admissions in 4% of the cohort. The incidence of severe maternal morbidity and mortality was higher during the Gamma wave than during the Delta wave (P=0.003). Vaccination conferred protection against the endpoint [relative risk (RR) 0.4, 95% confidence interval (CI) 0.1-0.9; P=0.0169]. Factors associated with severe morbidity and mortality included caesarean section (RR 3.7, 95% CI 1.7-7.9; P=0.0008), SARS-CoV-2 infection in the third trimester (RR 2.4, 95% CI 1.1-5.6; P=0.0006) and comorbidities (RR 3, 95% CI 1.8-5.2; P<0.0001). Conclusions: COVID-19 was significantly associated with the risk of severe maternal morbidity and mortality. Immunization of pregnant women against COVID-19 was highly protective against adverse outcomes, and should be encouraged during pregnancy.