2.1. Search Strategy

PubMed, Web of Science, Embase and Cochrane Library were searched from January 2020 to December 2022. The following combined search keywords were used in PubMed: “sFlt-1/PlGF” OR “Soluble fms-like tyrosine kinase-1” OR “Placental growth factor” AND “pregnancy” OR “pregnant” AND “COVID-19” OR “SARS-CoV-2” OR “severe acute respiratory syndrome coronavirus-2”. For other databases, the search terms were adapted correspondingly. For reports by the same author, only the latest or most intact were used to avoid the overlapping of queues. All references were imported into Endnote (ver. X9) and duplicates were removed before exporting them to the software-screening tool, Rayyan [20]. In addition, reference lists of relevant articles and systematic reviews were searched for further potential studies.