Transmission of the herpes simplex virus (HSV) to a baby can occur at three points: before birth (in utero), during delivery, or shortly after birth. While possible, transmission in the womb is the rarest pathway. The timing of the infection determines the medical condition and potential severity for the newborn. The most significant risk occurs during passage through the birth canal.
Transmission Before Birth
Transmission of HSV before birth is called in utero or congenital infection, accounting for only about 5% of all neonatal herpes cases. This rare event occurs when the virus travels from the mother’s bloodstream, crosses the placenta, and infects the fetus.
The risk is highest if the pregnant person acquires a primary, or first-time, HSV infection, especially during the first or second trimester. A primary infection means the mother has not yet produced protective antibodies to defend the fetus. If the mother has a recurrent infection, antibodies are present, making the chance of the virus spreading through the blood extremely low.
Outcomes of Congenital Infection
Congenital herpes, resulting from transplacental infection, can be severe because the virus interferes with fetal development. It affects developing organs and the central nervous system during early gestation. The effects of this prenatal exposure are often present at birth.
Developmental issues include microcephaly (abnormally small head) and severe brain damage. Ocular complications, such as chorioretinitis, are common and can lead to vision impairment, and some infants may have specific skin lesions or scars.
High Risk Transmission During Delivery
The most common scenario is transmission during the birthing process, known as perinatal transmission, accounting for approximately 85% of neonatal herpes cases. This occurs when the infant contacts the virus while passing through the infected birth canal. The virus may be present in active genital lesions or from asymptomatic viral shedding.
The risk is dramatically influenced by the timing of the mother’s infection. If the mother acquires a primary HSV infection late in the third trimester, the risk is highest (30% to 50%). This high rate occurs because the mother has not had time to develop and transfer protective antibodies to the baby.
In contrast, if the mother has a history of recurrent genital herpes, the transmission risk is significantly lower, typically less than 5%. Maternal antibodies developed from the previous infection cross the placenta, providing the newborn with protection.
The resulting condition, Neonatal Herpes, is categorized into three types:
- Disease localized to the skin, eyes, and mouth (SEM).
- Central nervous system (CNS) disease (inflammation of the brain and spinal cord).
- Disseminated disease, the most dangerous form involving multiple organs like the liver and lungs.
Reducing Transmission Risk During Pregnancy
Healthcare providers focus on proactive management to mitigate the risk of HSV transmission during pregnancy. Pregnant individuals should disclose any history of genital herpes to their obstetric team so appropriate preventive strategies can be implemented. The most effective intervention is antiviral suppressive therapy in the final weeks of pregnancy.
Antiviral medications, such as acyclovir or valacyclovir, are prescribed starting at or after 36 weeks of gestation. The purpose is to reduce recurrent outbreaks and decrease asymptomatic viral shedding at delivery. This therapy substantially lowers the chance that a Cesarean section will be necessary due to an active outbreak.
If a mother has active genital lesions or experiences prodromal symptoms (like vulvar pain or tingling) at the onset of labor, a Cesarean section is generally recommended. This surgical delivery prevents the baby from contacting infected secretions, dramatically reducing transmission risk. Avoiding new HSV exposure during pregnancy, particularly in the third trimester, is also advised to prevent high-risk primary infection.