CMV is the most common cause of congenital viral infection, occurring in approximately 1% of all newborns. Primary CMV infection occurs during pregnancy in 2% of women, with intrauterine transmission in approximately 40% of the cases. In almost all cases, the virus is acquired asymptomatically.
Testing for immunoglobulin G-specific antibodies can indicate which women have been infected in the past.
Diagnosis in the mother
CMV infection is asymptomatic in 90% of individuals, and when the signs are rather non-specific but include:
It is recommended that diagnosis is made in clinically suspected cases using serology, with seroconversion of CMV-specific immunoglobulin G (IgG) from a current and previously saved serum sample (usually from UK pregnancy booking bloods) as a confirmed acute infection.
Without a previous sample to compare to, the presence of CMV-specific immunoglobulin M can increase suspicion of an acute infection, however if may remain positive for over a year from the initial infection. There is also the possibility of false positive IgM findings, particularly if a secondary infection.
IgG avidity index tests are a more recent advance that can potentially help distinguish how recent a CMV infection occurred, as the lower the avidity index the more recent the CMW infection is likely to be.
Diagnosis in the fetus
Ultrasound is a non-invasive but poorly sensitive test, identifying around 20% of infected infants. Magnetic resonance imaging (MRI) can also aid the diagnosis via imaging.
Markers suggestive of CMV infection include:
- cerebral ventriculomegaly
- echogenic fetal bowel
- fetal growth restriction
- periventricular calcifications
- periventricular (pseudo) cysts
- pleural effusion.
CMV can also be diagnosed based on culture and polymerase chain reaction (PCR) on amniotic fluid, and an amniocentesis is recommended to be performed at least 7 weeks after the presumed time of maternal infection, and after 21 weeks gestation. Performing this test too soon can increase the chance of a false negative as the fetus hasn’t matured enough to excrete the CMV.
It is worth remembering that positive amniotic fluid results do not directly correlate to those babies that will be symptomatic at birth.