Congenital chickenpox is not inevitable when maternal chickenpox occurs in the 21 days preceding parturition. The incubation period of congenital chickenpox, defined as the interval between onset of rash in the mother and onset in the fetus or neonate, is usually 9 to 15 days. This interval is slightly shorter than the normal postnatal incubation period, suggesting that the infecting transplacental dose of virus may be higher than which occurs postnatally. Very rarely, presumably when fetal infection is due to the pimary maternal viremia, the exanthem appears in the mother and neonate within 3 days of each other or even simultaneously. In contrast to postnatally acquired neonatal chickenpox, congenital chickenpox is associated with significant mortality.
Severe cases resemble varicella in the immunocompromised host.
The spectrum of illness also includes extremely mild infections with only a handful of lesions. When varicella-zoster infection occurred more than 1 week before delivery, antibody titers in maternal blood and cord blood were the same. In contrast, when infection occurred 3 to 5 days before delivery, maternal antibody was present at parturition and the varicella-zoster antibodies in the neonate was either absent or at least eightfold lower. These data suggest that a lag of several days occurs before IgG antibodies to varicella-zoster virus cross the placenta and equilibrate with the fetal circulation. The development of mild congenital varicella in the presence of placentally transferred maternal antibody has been demonstrated.