Conclusions and Relevance  In this cohort study, there was no evidence of placental infection or definitive vertical transmission of SARS-CoV-2. Transplacental transfer of anti-SARS-CoV-2 antibodies was inefficient. Lack of viremia and reduced coexpression and colocalization of placental angiotensin-converting enzyme 2 and transmembrane serine protease 2 may serve as protective mechanisms against vertical transmission. https://web.archive.org/web/20201222162005/https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2774428