Since the incubation period for varicella is 10 to 21 days, those cases beginning in the first 10 days of life are considered to have been acquired in utero. The prognosis, however, differs markedly between those cases in which maternal illness began 5 or more days from delivery and those in which maternal illness occured 4 days before to 1 day after delivery. In the first group, neonatal disease usually begins with the first 4 days of life, and the prognosis is good. Presumably, maternal immunity has appeared before delivery and has been transferred to the baby before birth. In the second group, neonatal disease begins between 5 and 10 days after delivery.
Placenta acts as a partial barrier to infection at term as well as earlier during pregnancy.
Only about one in six such maternal infections result in neonatal disease. The precise risk of horizontal transmission in maternity wards or in the newborn nursery after the virus has been introduced is unclear, but apparently it is low. The percentage of susceptible persons among women raised in tropical climates is somewhat higher. Because IgG antibodies to the varicella-zoster virus cross the placenta, the newborns of immune mothers should likewise be at least partially protected. Nevertheless, perinatal chickenpox has been reported in infants born to women with positive history of chickenpox.