Perinatal Transmission of HSV
How much risk is there of perinatal transmission of HSV ? Does it always gets transmitted from mother to child?
@claudio HSV can be vertically transmitted to the infant before, during, or after delivery, although intrapartum transmission accounts for most cases. Maternal age of less than 21 years is a risk factor for vertical transmission.
Approximately 5% of all cases of neonatal HSV infection result from in utero transmission. With primary infection, transient viremia occurs. HSV has the potential for hematogenous spread to the placenta and to the fetus. Hematogenous spread can produce a spectrum of findings similar to other TORCH (toxoplasmosis, other infections, rubella, cytomegalovirus, and herpes simplex) infections, such as microcephaly, microphthalmia, intracranial calcifications, and chorioretinitis.
Intrapartum transmission accounts for most neonatal infections and occurs with passage of the infant through an infected birth canal. The use of a fetal-scalp electrode increases the risk for intrapartum transmission.  From 75% to 90% of infants with neonatal HSV are born to infected asymptomatic mothers who have no known history of genital HSV.
Postnatal transmission of HSV can occur through contact with infected parents or health care workers.
Perinatal transmission rates
Intrapartum transmission rates depend on the type of clinical HSV infection. Lower rates are noted in the presence of protective maternal antibodies that cross the placenta. See the following:
Primary HSV infection - Transmission rate of 50%
Nonprimary first-episode infection - Transmission rate of 33%
Recurrent infection or asymptomatic shedding - Transmission rate of 0-4%
The overall chance of neonatal infection from asymptomatic shedding in a woman with a history of genital HSV infection is estimated to be less than 4 in 10,000 (ie, 1% risk of asymptomatic shedding multiplied by the [up to] 4% risk of transmission).
To determine the frequency and sequelae of HSV shedding at the time of delivery, Brown et al obtained HSV cultures (from both cervix and external genitalia) within 48 hours of delivery in 40,023 women.  HSV was isolated in 202 women (0.5%), of whom approximately one half had no prior history of genital HSV. Serology was also available in 177 of the 202 cases (see image below). Based on the serology of these 177 women, 26 (15%) had first-episode disease, and 151 (85%) had recurrent infection.