The herpes simplex virus (HSV), which causes oral cold sores (HSV-1) and most genital herpes (HSV-2), is a common infection that establishes a lifelong, latent presence in nerve cells. Expectant mothers who have the virus frequently ask whether pregnancy increases the likelihood of an outbreak. Although the virus remains dormant for long periods, it can reactivate, causing painful sores or asymptomatic shedding (release of the virus without visible symptoms). Understanding the biological changes of pregnancy is necessary to address the risk of viral reactivation and ensure a safe delivery.
Hormonal and Immune Effects on Outbreak Frequency
Pregnancy introduces alterations that influence the herpes virus’s tendency to reactivate. A major factor is the shift in the maternal immune system, which undergoes mild, temporary suppression to prevent the body from rejecting the developing fetus. This reduction in immune response can lessen the body’s ability to keep latent HSV in check, increasing the chance of viral reactivation and an outbreak.
Dramatic fluctuations of hormones, particularly sustained high levels of progesterone, also play a role. Progesterone may modulate the immune environment, making the body more vulnerable to infections, including HSV. This hormonal environment, combined with the physical and emotional stress of pregnancy, can trigger the virus to emerge from its latent state and travel to the skin surface.
Approximately 75% of women with a history of herpes may experience a recurrent outbreak during pregnancy, though severity and frequency vary widely. It is important to distinguish this from a primary, or first-time, infection during pregnancy. A first-time infection is often more severe and carries a greater risk of complications than a typical recurrence.
Risk of Transmission to the Infant
The risk of HSV transmitting to the infant, resulting in neonatal herpes, depends heavily on the timing of the mother’s infection. Neonatal herpes is a serious, potentially life-threatening condition that can lead to central nervous system damage. The risk is extremely low (less than 1%) for mothers who acquired genital herpes before pregnancy because protective antibodies cross the placenta and provide the baby with temporary immunity.
The most significant risk of transmission (30% to 50%) occurs when a mother acquires a primary genital HSV infection late in the third trimester. In this scenario, the mother’s immune system has not had enough time to produce and transfer sufficient protective antibodies to the fetus before delivery. Without these antibodies, the newborn is highly vulnerable to infection if exposed to the virus in the birth canal.
The virus can be transmitted to the infant through three main routes:
- Peripartum: The majority of cases (about 85%) occur during this period as the baby passes through the birth canal where the virus may be actively shedding.
- Postnatal: This accounts for about 10% of cases and happens after birth, often through contact with someone who has an active oral cold sore.
- In utero: This is the least common route (about 5% of cases), where the virus crosses the placenta.
Antiviral Strategies During Pregnancy and Delivery
Managing herpes during pregnancy centers on minimizing the risk of viral shedding around the time of delivery to protect the infant. Antiviral medications like acyclovir and valacyclovir are commonly used and considered safe throughout pregnancy, as studies have not demonstrated them to be teratogenic. These medications interfere with the virus’s ability to replicate, reducing the duration and severity of outbreaks.
Suppressive therapy is a standard preventative measure, involving a daily oral dose of antiviral medication starting around 36 weeks of gestation and continuing until delivery. This treatment is highly effective, reducing the frequency of recurrent outbreaks at the time of labor and decreasing the rate of asymptomatic viral shedding. The goal is to prevent the need for a cesarean delivery by ensuring the birth canal is virus-free.
The mode of delivery depends on the presence of active lesions or prodromal symptoms at the onset of labor. A vaginal delivery is safe if the mother has a history of recurrent genital herpes but has no visible lesions or symptoms. If active lesions or prodromal symptoms (such as tingling or nerve pain) are present, a cesarean delivery is recommended. This bypasses the infected birth canal and prevents the infant’s exposure to the virus, greatly reducing the risk of transmission.