Women who took medication to treat herpes infections during pregnancy weren't more likely to have a baby with birth defects than women who didn't take these drugs in a study of over 800,000 babies born in Denmark.

While the finding doesn't rule out a link between these antivirals and specific types of birth defects, it does show that the drugs aren't causing a lot of birth defects, said Dr. James Mills.

Some medications, including some seizure and acne drugs, have been linked to higher rates of birth defects - a famous side effect of the drug thalidomide that was used in the 1960s to combat morning sickness in pregnant women. But not much was known about the effects of antiviral drugs used to treat herpes.

As with many drugs used during pregnancy there wasn't much evidence to show they were safe, Mills, an epidemiologist at the National Institutes of Health in Bethesda, Maryland, told Reuters Health. Even though there wasn't a major concern about these drugs, medication use during pregnancy is always an issue, he said. Mills is co-author of an editorial accompanying the Danish study, which is published today in the Journal of the American Medical Association.

Herpes viruses hide out in nerve cells in the body where they periodically cause outbreaks of painful conditions, such as cold sores, genital herpes and, later in life, skin blisters called shingles. The U.S. Centers for Disease Control and Prevention reports that 1 in 6 Americans between 14 and 49 years old is infected with genital herpes, and one study showed that 1 in 4 people will get shingles at some point.

These infections are typically treated with antiviral drugs such as acyclovir (Zovirax). While herpes can't be cured by medication, antivirals can help control the symptoms and outbreaks and relieve pain.

Dr. Bjorn Pasternak and Dr. Anders Hviid, epidemiologists at the Statens Serum Institut in Copenhagen, Denmark, looked at the records of all babies born in Denmark between 1996 and 2008 - almost 840,000 births. They also examined data from nationwide registries that keep track of all diagnoses of major birth defects and all prescriptions filled at pharmacies in Denmark.

Comparing these registries, the investigators were able to figure out when in the course of their pregnancies women had filled prescriptions for (and presumably taken) herpes medication, and which women gave birth to babies with birth defects.

Pasternak and Hviid focused on medication that pregnant women took during their first trimester, the window of time when organs are developing and most susceptible to drug effects.

During the 12-year study, about 1,800 babies were born to mothers who filled prescriptions for acyclovir, valacyclovir (Valtrex), and famciclovir (Famvir) during their first trimester. Forty of those babies had birth defects - 2.2 percent of them. In comparison, close to 20,000 of the 836,000 babies whose mothers didn't take those drugs during the first trimester had birth defects, or 2.4 percent.

Women who filled prescriptions for antivirals during their second and third trimesters also weren't any more likely to give birth to a baby with birth defects than women who didn't fill these prescriptions.

Despite the size of the study, the authors did not have enough data to determine whether there might be a link between any of these drugs and specific birth defects.

Pasternak said more research is needed to rule out a link between herpes medications and specific birth defects, and to answer the question of whether these drugs affect pregnancy complications such as spontaneous abortion.

But, he told Reuters Health, from a public health perspective it is important to know that women who take antivirals early in pregnancy are not placing themselves at risk of having a child with a birth defect on the whole.

While the study can't rule out whether an individual taking a herpes antiviral drug might put her baby at risk for certain kinds of defects, this is the best we can do at the moment, Pasternak said.

The study was funded by the Danish Medical Research Council and the Lundbeck Foundation, which invests in pharmaceutical companies.

SOURCE: link.reuters.com/xyg86n JAMA/Journal of the American Medical Association, August 25, 2010.

Medicine