What are the dangers, if any, during pregnancy if one has herpes? Is there a possibility of passing herpes to the fetus or of the fetus having birth defects?

B.J.

Genital and oral herpes are infections by the herpes simplex viruses, referred to as HSV-1 and HSV-2. As many as 25 percent of women of childbearing age may have been infected with HSV-2, the primary cause of genital herpes. (For more information on these viruses and how they are transmitted, please see my previous column on genital herpes.) It is possible for a pregnant woman who has genital herpes to infect her baby. Usually this occurs at the time of delivery as the baby passes through the birth canal, but in rare cases, it can occur during gestation, if the infection spreads from the vagina to the womb. Though it is rare, infection of the fetus in the uterus can cause early miscarriage or birth defects such as eye problems and abnormalities of the brain and spinal cord. More commonly, the baby is infected at the time of delivery. This results in symptoms days to a few weeks after birth. The spectrum of disease varies from a few blisters on the baby's skin to a severe widespread illness involving the brain and internal organs. Such infection can result in death.

The overall risk of a newborn being born with or developing herpes is between 1 in 2,000 and 1 in 10,000 births. A woman who acquires herpes a short time before delivery is at much higher risk of passing it to her baby than if she was infected early in pregnancy or if she has a recurrence of her herpes at the time of delivery. This is probably because the quantity of virus is highest with a recent first infection. One study estimates the risk to the fetus to be almost 50 percent if the woman has her first herpes outbreak with an active lesion during delivery. In other words, the baby has a 1 in 2 chance of being infected in that circumstance. For women who have a recurrent outbreak and an active lesion at delivery, the risk is about 4 percent, or 1 chance in 25. If the woman is having a recurrence without noticeable symptoms (but still shedding the virus) the risk is less than 0.1 percent, or 1 chance in a 1,000.

In general, any woman who has not had herpes and who has a sexual partner with herpes is considered at high risk. Experts recommend that such women abstain from sex -- or at least use condoms -- in the second half of pregnancy. Cesarean section clearly lowers the risk of infection, but there is some controversy over when to recommend it. Before delivery, physicians should question women about symptoms of active herpes infection and examine them for any signs. In most if not all cases of active herpes, the doctor should recommend C-section. After a baby is born to a woman with herpes, the newborn should be closely monitored for any sign of disease. At the first sign of infection, the baby should receive the antiviral drug acyclovir (trade name Zovirax).

In some medical centers, women with a history of genital herpes are tested for HSV at the time of delivery. However, it is unclear what to do if the mother's HSV cultures are positive. By the time the results are back from the lab, the baby has already been born and it is too late to decide to perform a C-section. And experts generally do not recommend giving acyclovir to every baby born to a mother with positive cultures, because most such infants will not develop disease even without treatment.