Cytomegalovirus (CMV) is one of the herpes group of viruses. It is one of the major congenital causes of deafness, blindness and mental impairment; a fact made even more shocking because so few pregnant women have heard about it. The good news is that many women (40-80% in Europe) are immune to it through previous infection, in which case it is highly unlikely to cause an infection of the developing baby. The bad news is that if you''re not immune, and catch it in pregnancy, there is a 50/50 chance of passing it to the baby.