Varicella zoster virus (VZV) causes the illness known as chicken pox. Although most (85-90%) pregnant women have already been exposed to the virus and therefore are immune, some may not have previously had the infection. Since the virus can cause birth defects or illness in the baby (depending on when during the pregnancy an infection occurs), testing is available pre-conception or in early pregnancy to determine if the woman has antibodies to VZV. If she doesn’t and is therefore not immune, a vaccine can be given before the woman gets pregnant; if she is already pregnant and may have been exposed to the virus, treatment is available that can prevent or weaken the severity of the illness.