Neonatal herpes simplex virus (HSV) infection most commonly occurs when the virus is present in the birth canal of an asymptomatic mother during labor and delivery. The virus may be present as a result of reactivation of prior infection or infection acquired at some time during the pregnancy. The consequences of neonatal infection with HSV include neurodevelopmental disability and death. It has also been noted that genital HSV infection acquired during pregnancy may result in pre-term labor, intrauterine growth retardation and spontaneous abortion. Brown and colleagues used virologic and serologic methods to determine the rate of acquisition of HSV infection among pregnant women.

Serum samples for antibody to HSV type 1 and type 2 were obtained at the first prenatal visit in 8,538 women. In addition, serum samples were obtained at 14 to 18 weeks of gestation, at 24 to 28 weeks of gestation and at the time of labor. Cultures for HSV were obtained from the external genitalia and from the cervix of these women at the onset of labor. The serum samples from each of the women were tested simultaneously using Western blot assays. The medical records of women who became seropositive for HSV during the pregnancy were reviewed for symptoms of genital herpes during pregnancy.

Of the approximately 8,500 women included in the study, 24 percent were HSV-negative at entry into the study; 48 percent were seropositive for HSV type 1 antibody; 11 percent were seropositive for HSV type 2 antibody; and 17 percent were seropositive for both types of antibodies. During the course of pregnancy, a total of 94 women became seropositive for HSV. After adjustment for a 40-week gestation period, this total represents 2 percent of all susceptible patients. Among the 94 women who became seropositive during the prenatal period, 60 had subclinical infections. Thirty percent acquired the infection in the first trimester, 30 percent in the second trimester, and 40 percent in the third trimester. Thirty-four of the newly infected patients had symptomatic infections. Twenty-one percent of infections occurred in the first trimester, 44 percent in the second trimester, and 35 percent in the third trimester. There were no cases of neonatal herpes and there was no increase in pregnancy-related morbidity among the 94 women who seroconverted.

Nine other women who were diagnosed with genital herpes obtained the infection near the onset of labor; five of these women had lesions at the time of labor. Neonatal infections occurred in four of the nine infants born to these women.

The authors conclude that infection with HSV during the prenatal period did not increase the frequency of pregnancy-related complications if seroconversion was complete before the onset of labor. Approximately 2 percent of susceptible women may acquire HSV infection during pregnancy. However, only an infection obtained near the time of labor and delivery appears to be associated with neonatal infection and accompanying morbidity and mortality. The latter part of pregnancy may be the best time to consider serologic testing for HSV and to recommend preventive methods to susceptible mothers, such as the use of condoms or sexual abstinence.