Herpes Simplex Virus (HSV) is a common infection with two types: HSV-1, often associated with oral cold sores, and HSV-2, which is the more frequent cause of genital herpes. Both types can be transmitted to a developing fetus or newborn. While the virus may cause mild or asymptomatic infections in adults, exposure during pregnancy carries a risk of serious outcomes for the infant. The risk to the infant depends heavily on the timing of the infection and the mother’s immune response.
The Risk of Congenital Birth Defects
The possibility of herpes causing a congenital birth defect involves the rare event of the virus crossing the placenta and infecting the fetus in utero. This transplacental transmission is estimated to account for only about 5% of all neonatal herpes cases. When this occurs, it is most often associated with a maternal primary infection during the first 20 weeks of gestation. The developing fetal organs and central nervous system are particularly vulnerable during this early stage of pregnancy.
Congenital HSV infection can result in severe structural abnormalities and developmental issues. These changes may include microcephaly (smaller than normal head size) and hydrocephalus (accumulation of fluid in the brain). Ocular anomalies like chorioretinitis, an inflammation of the eye’s retina and choroid, may also be present at birth. Distinctive scarring or raw skin lesions are sometimes seen in infants with congenital HSV exposure.
Transmission Timing and Risk Factors
The risk of transmission is highly dependent on whether the mother is experiencing a primary (first-time) infection or a recurrent outbreak. A primary HSV infection acquired late in the third trimester poses the greatest danger to the newborn, with transmission rates ranging from 30% to 50% without intervention. This elevated risk occurs because the mother’s body has not had enough time to produce protective antibodies that could cross the placenta and shield the baby.
Conversely, a recurrent outbreak, where the mother has pre-existing antibodies, carries a much lower transmission risk, typically less than 1% to 3%. The antibodies pass to the fetus through the placenta, offering a degree of passive immunity against the virus. The vast majority of infant infections occur when the virus is acquired during passage through the birth canal, known as intrapartum transmission.
Neonatal Herpes: A Separate, Urgent Risk
Neonatal herpes refers to infection acquired during or immediately after birth, and it represents the most common form of severe infant infection. This condition is categorized into three main types based on the affected areas.
The first is disease localized to the skin, eye, and mouth (SEM), which accounts for about 45% of cases. These infants typically develop characteristic fluid-filled blisters on the skin, often appearing in the first one to two weeks of life.
The central nervous system (CNS) form involves the brain and spinal cord, making up approximately 30% of cases. Symptoms are often non-specific, presenting as irritability, lethargy, poor feeding, or seizures, sometimes without the tell-tale skin lesions.
The most dangerous form is disseminated disease, accounting for about 25% of cases, where the virus has spread to multiple organs, including the lungs and liver. If left untreated, neonatal herpes has a high fatality rate, and even with prompt antiviral treatment, survivors often face long-term neurological impairment.
Managing Herpes During Pregnancy
Medical management focuses on preventing the virus from reaching the newborn, primarily by reducing the chance of an active outbreak at the time of delivery. Pregnant individuals with a history of recurrent genital herpes are often prescribed suppressive antiviral therapy starting at 36 weeks of gestation. Medications such as acyclovir or valacyclovir are used to decrease the frequency of outbreaks and minimize asymptomatic viral shedding in the genital tract. This prophylactic treatment significantly lowers the need for a Cesarean delivery.
The mode of delivery is carefully chosen based on the mother’s status at the onset of labor. A Cesarean section is the recommended delivery method if the mother has active genital lesions or prodromal symptoms, such as vulvar pain, at the time of labor. This prevents the infant from direct contact with viral shedding. For a mother with a history of recurrent herpes who has no active lesions at the time of labor, a vaginal delivery is considered safe.