TUESDAY, Aug. 24 (HealthDay News) -- Pregnant women who take antiviral drugs against the herpes virus should not worry that their use will cause birth defects, Danish researchers report.

More than 1 percent of women develop herpes simplex during the first trimester of pregnancy. And while the use during pregnancy of antiviral drugs such as acyclovir (Zovirax), valacyclovir (Valtrex) and famciclovir (Famvir) are believed to be safe, data on their use in early pregnancy has been limited, the researchers noted.

But the new analysis suggests that "pregnant women needing treatment for a herpes infection in the first trimester can use antivirals without concern about birth defect risk," said lead researcher Dr. Bjorn Pasternak, from the department of epidemiology research at the Statens Serum Institut in Copenhagen.

Acyclovir was the most commonly used antiviral, and should be the antiviral drug of choice in the treatment of herpes infections in early pregnancy, Pasternak said.

"More research is however needed with regard to use of antiviral drugs in pregnancy and the risk of other adverse fetal outcomes, such as growth restriction, preterm birth and spontaneous abortion," he said.

The report, funded by the Danish Medical Research Council and the Lundbeck Foundation, is published in the Aug. 25 issue of the Journal of the American Medical Association.

For the study, Pasternak and colleague Dr. Anders Hviid, also from the Statens Serum Institut, collected data on almost 840,000 births in Denmark from January 1996 to September 2008.

The women in the study had no chromosomal abnormalities, genetic syndromes, birth defect syndromes, or congenital viral infections, the researchers noted.

Among the more than 1,800 women exposed to acyclovir, valacyclovir or famciclovir during their first trimester, 40 infants (2.2 percent) had a major birth defect, compared with 19,920 infants (2.4 percent) born to women not exposed to these drugs, the researchers found.

Based on these numbers, Pasternak and Hviid concluded that the antiviral drugs were not associated with birth defects.

For women taking acyclovir, the most commonly prescribed antiviral, there were 32 cases of birth defects among 1,561 exposed to the drug (2 percent), compared with 2.4 percent of the birth defects among women not exposed acyclovir, they noted.

Among women taking valacyclovir there were seven cases of birth defects among 229 infants (3.1 percent), a rate that was not significantly higher than that of women not exposed to the drug. For famciclovir, there was one case of a major birth defect among the 26 women taking the drug (3.8 percent), but the researchers note that the use of famciclovir was uncommon.

Further analysis did not find any associations between these antiviral drugs and 13 different subgroups of birth defects. However, Pasternak and Hviid stressed that the number of exposed cases in each subgroup was small.

"Genital herpes is a common sexually transmitted disease in women of reproductive age. Its prevalence in women is actually higher than in men," noted Dr. Jennifer Wu, an obstetrician/gynecologist at Lenox Hill Hospital in New York City. She called the findings "reassuring for patients who may need to use the medications in the first trimester."

Dr. James L. Mills, an investigator in the Epidemiology Branch of the Eunice Kennedy Shriver National Institute of Child Health and Human Development and coauthor of an accompanying journal editorial comment that, "herpes viruses are an important problem during pregnancy. Many women are treated but there was, until now, only limited information regarding the possible teratogenicity [ability to harm the fetus] of the treatment."

The Danish study answers one key question: are acyclovir and related compounds adding significantly to the number of children born with birth defects? "The answer is no," he said.

More information

For more information on pregnancy, visit the U.S. National Institutes of Health.