A study published in the Journal of the American Medical Association claims that antivirals are safe to prescribe to women in the first trimester of pregnancy.

In the national registry-based study, the link between exposure to aciclovir, valaciclovir, and famciclovir in the first trimester of pregnancy and major birth defects was investigated. The team also examined risks in subgroups of major birth defects by organ system.

A total of 837,795 infants were born over the 13-year study period. Among 1,804 pregnancies exposed to aciclovir, valaciclovir, or famciclovir in the first trimester, 40 infants (2.2%) were diagnosed with a major birth defect compared with 19,920 (2.4%) among those not exposed. The team also found no associations between antiviral drug exposure and 13 different subgroups of birth defects, although the number of exposed cases in each subgroup was small.

The authors commented that their study "may support informed decisions on safety when prescribing antivirals for herpes infections in early pregnancy" and one added:  "Aciclovir is the most extensively documented antiviral and should therefore be the drug of choice in early pregnancy, while data on valaciclovir and famciclovir are still insufficient."