Placental barrier against COVID-19.

Vertical transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and possible induction of pregnancy complications, including miscarriage, fetal malformations, fetal growth restriction and/or stillbirth, are serious concerns for pregnant individuals with COVID-19. According to clinical information, the incidence of vertical transmission of SARS-CoV-2 is limited to date. However, even if a neonate tests negative for SARS-CoV-2, frequent abnormal findings, including fetal and maternal vascular malperfusion, have been reported in cases of COVID-19-positive mothers. Primary receptor of SARS-CoV-2 is estimated as angiotensin-converting enzyme 2 (ACE2). It is highly expressed in maternal-fetal interface cells, such as syncytiotrophoblasts, cytotrophoblasts, endothelial cells, and the vascular smooth muscle cells of primary and secondary villi. However other route of transplacental infection cannot be ruled out. Pathological examinations have demonstrated that syncytiotrophoblasts are often infected with SARS-CoV-2, but fetuses are not always infected. These findings suggest the presence of a placental barrier, even if it is not completely effective. As the frequency and molecular mechanisms of intrauterine vertical transmission of SARS-CoV-2 have not been determined to date, intensive clinical examinations by repeated ultrasound and fetal heart rate monitoring are strongly recommended for pregnant women infected with COVID-19. In addition, careful investigation of placental samples after delivery by both morphological and molecular methods is also strongly recommended.