Acyclovir (ACV), an antiviral nucleoside analog, is active against Herpes simplex viruses (HSV1, HSV2) and varicella virus (VZV). These viruses seems to be prejudicial to the pregnant woman and to the fetus. Yet, ACV is not recommended for use in pregnancy. However in certain cases, this drug has been used. We review in this paper, the pharmacokinetics and transplacental passage of ACV, indications, and whether the benefits of the administration of ACV in pregnancy outweigh the theoretical risks. Peak and trough plasma concentrations of ACV in pregnant women seem to be lower as compared to those of non-pregnant adults but effective. This drug crosses the placenta. Levels of ACV in cord blood ranged from 0.5 to 3 mumol/l. In as much as in vitro inhibitory doses 50 (ID 50) for HSV1, HSV2 and VZV ranged from 0.1 to 3 mumol/l, it is quite likely that levels noted above may be effective for in utero inhibition of viral replication. No adverse effects were noted in newborn exposed in utero to ACV. But one must be careful about the direct effects of this drug on nucleic acid metabolism despite encouraging results on animal fetuses. Based on these findings and from our experience, ACV can be administered in pregnancy in two particular situations: in cases of maternal severe viral infections and in order to inhibit in utero VZV replication. Doses required for pregnant women range from 5 to 15 mg/kg/8 hours given intravenously, and 200 mg of oral Acyclovir 5 times daily.