Cytomegalovirus (CMV) is a common herpesvirus that most women contract unknowingly before reaching child-bearing age. It’s usually harmless except during pregnancy when, if passed on to the developing fetus, it is a leading cause of miscarriage and birth defects, including cerebral palsy and hearing loss. Researchers have long known that the risk for complications is particularly high for women infected by CMV for the first time during pregnancy, but they haven’t fully understood why those who already carry the virus are less vulnerable.
