Objective: To describe and compare measures of maternal depression, anxiety, and posttraumatic stress symptoms before and during the coronavirus disease 2019 (COVID-19) pandemic in a Brazilian birth cohort. Methods: All hospital births occurring in the municipality of Rio Grande (southern Brazil) during 2019 were identified. Mothers were invited to complete a standardized questionnaire on sociodemographic and health-related characteristics. Between May and July 2020, we tried to contact all cohort mothers of singletons, living in urban areas, to answer a standardized web-based questionnaire. They completed the Edinburgh Postnatal Depression Scale (EPDS) and Generalized Anxiety Disorder 7-item (GAD-7) in both follow-ups, and the Impact of Event Scale (IES) in the online follow-up. Results: We located 1,136 eligible mothers (n=2,051). Of those, 40.5% had moderate to severe stress due to the current pandemic, 29.3% had depression, and 25.9% had GAD. Mothers reporting loss of income during the pandemic (57.2%) had the highest proportions of mental health problems. Compared to baseline, the prevalence of depression increased 5.7 fold and that of anxiety increased 2.4-fold during the pandemic (both p o 0.001). Conclusion: We found a high prevalence of personal distress due to the ongoing COVID-19 pandemic, and a clear rise in both maternal depression and anxiety.