Cytomegalovirus (CMV) is a herpes virus ubiquitous in humans, and it is the leading infectious cause of congenital malformations.1 Anti-CMV IgG and IgM reactive samples may indicate an acute, recent or reactivated infection.
Since fetal symptomatic congenital infection is mostly due to intrauterine transmission following primary maternal infection, differential diagnosis of primary versus recurrent infection, unspecific IgM or persistence of CMV-specific IgM antibody is crucial for the management of the pregnancy.2 Antibodies produced at an early stage during primary response have lower antigen avidity than those produced at a later stage.2
A low-avidity anti-CMV IgG detected before the 16th – 18th week of pregnancy, together with a positive anti-CMV IgM, is strong evidence of a recent primary infection, whereas a high avidity index during the first 12 – 16 weeks would be considered a good indicator of past infection.2 A high avidity result later in gestation cannot rule out a primary infection at an earlier stage of the pregnancy.2