Mpox (Monkeypox) in Pregnant Women, the Placenta and Fetus: Correlation with Maternal-Fetal Transmission, Pathology and Strain Differences from MPXV Clades Ia, Ib, IIa, and IIb.

Since the elimination of smallpox, mpox (monkeypox) is the most medically significant orthopoxvirus infection. As a result of numerous regional, national and global outbreaks of MPXV (mpox virus), there is an abundance of new data available on the effects of the different viral clades on clinical obstetrical and perinatal outcomes when infection occurs in pregnancy. In addition, there have been additional placentas from cases of congenital MPXV infection available for study. These recent data indicate that there are prominent differences between viral strains and their effects on the fetus, with MPXV Clade I strains (Ia, Ib) having the greatest risk for an adverse outcome in pregnancy, and Clade II strains (IIa, IIb) having far less risk. In particular, the ongoing outbreak of MPXV Clade Ib in the DRC indicates that there is a significant risk for adverse perinatal outcomes associated with infection in pregnancy, especially during the first trimester. These outcomes include spontaneous abortion, stillbirth, neonatal death and congenital mpox. The placenta in cases of congenital infection demonstrates abundant virus in the chorionic villi, with prominent involvement of Hofbauer cells. Similar to smallpox, transplacental transmission and adverse pregnancy outcomes are an important feature of certain strains of this orthopoxvirus infection when occurring in pregnant women.