A number of factors determine the probability that a herpes simplex virus (HSV) infection will be transmitted from a mother to an infant in the perinatal period.
Factor
Significance
type of maternal infection (primary vs recurrent with lesions vs recurrent with asymptomatic shedding)
primary > recurrent with lesions > asymptomatic shedding
quantity of viral shedding in the birth canal (number of vesicles)
The greater the number of viral particles the greater the chance of transmission.
duration of time between rupture of the membranes and delivery
Risk increases as the interval from membrane rupture increases.
status of maternal antibodies to HSV
Risk reduced if specific antibodies are present.
use of fetal scalp electrodes or fetal instrumentation
Breech of the infant's mucocutaneous barriers offers portal for entry.
method of delivery (Cesarean section vs vaginal delivery)
Rate after Cesarean section less than with vaginal delivery in women with primary infection or recurrent infection with clinical vesicles.
The highest rate of transmission is seen in primary HSV infections acquired late in pregnancy.
• In the absence of protective antibodies maternal viremia can result in hematogenous spread to the fetus; this may explain the cases of neonatal HSV infection seen in children born by Cesarean section prior to rupture of the membranes.
• In addition, viral shedding in the birth canal is very high; vaginal delivery during an active primary infection results in a 30-50% transmission to the fetus.
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