Cytomegalovirus (CMV) is a highly common virus belonging to the herpes family, infecting most of the global population at some point. For most healthy adults and children, the infection is typically harmless, often causing no symptoms or a very mild, flu-like illness. Once infected, the virus remains in the body in a latent state. The primary concern arises when a pregnant woman acquires the infection for the first time, as this primary infection carries the risk of being passed to the developing fetus, resulting in congenital CMV infection.
Global and Regional Prevalence Statistics
The prevalence of CMV during pregnancy is determined by two statistics: the number of women previously exposed and the rate of new infections during gestation. Globally, the estimated seroprevalence—the percentage of women of childbearing age who have been previously infected and carry antibodies—is approximately 86%. This high rate signifies that most pregnant women already possess some level of immunity before conception.
These seroprevalence rates vary significantly by region. Rates in the World Health Organization’s European region are around 66%, while those in the Eastern Mediterranean region can exceed 90%. In the United States, seroprevalence is often correlated with socioeconomic status and race, ranging from 40% to 60% in some groups to over 80% in others.
The most significant risk to the fetus is associated with a primary infection, which is the first-time exposure to the virus during pregnancy. The rate of this primary infection in susceptible, or seronegative, pregnant women ranges from about 0.7% to 4.1%. While women with pre-existing immunity can experience a recurrent infection, the risk of the virus transmitting to the fetus and causing severe harm is considerably lower compared to a primary infection.
Understanding Transmission Routes and Prevention Strategies
CMV is transmitted through direct contact with infected bodily fluids, including saliva, urine, tears, blood, and breast milk. Because the virus can shed in these fluids for months or years after infection, young children are considered the most common source of transmission to pregnant women.
The virus does not spread through casual contact, making specific hygiene practices highly effective in reducing risk. Pregnant women should wash their hands thoroughly with soap and water, especially after changing diapers or helping a child use the toilet. Handwashing is also necessary after wiping a child’s nose or handling toys and surfaces that may be contaminated with saliva or urine.
Practical behavioral changes focus on minimizing contact with a child’s oral secretions. It is recommended to avoid sharing food, drinks, eating utensils, or toothbrushes with young children. Avoid putting a child’s pacifier in her own mouth or kissing a child on the lips. Kissing a child on the forehead or cheek is a safer alternative that reduces the risk of saliva-to-skin contact.
Potential Health Consequences for the Fetus
When CMV is transmitted from the mother to the fetus during pregnancy, it is known as congenital CMV infection, the most common viral cause of birth defects. The severity of the outcome is highly dependent on the timing of the maternal infection during gestation. Although the rate of transmission from mother to fetus is highest during the third trimester, the risk of the fetus developing severe long-term complications is greatest following a primary infection in the first trimester.
Around 90% of babies born with congenital CMV infection appear completely healthy at birth. However, approximately 10% to 15% of these initially asymptomatic infants may still develop long-term problems, most commonly sensorineural hearing loss (SNHL). This hearing loss can be progressive and may not be detected until months or years after birth, requiring long-term monitoring.
The remaining 10% of infected infants are symptomatic at birth and present with severe clinical signs. These symptoms and neurological findings include:
- Microcephaly (abnormally small head size)
- Jaundice
- Hepatosplenomegaly (enlarged liver and spleen)
- Petechiae (a purplish-red rash)
- Vision loss due to retinitis
- Developmental delays
- Seizures
- Intellectual disability
Screening and Diagnostic Procedures
Diagnosis typically begins with maternal blood tests that check for two types of antibodies: Immunoglobulin M (IgM) and Immunoglobulin G (IgG). A positive IgG result indicates past exposure and immunity, while a positive IgM suggests a recent or active infection. Because IgM can remain positive for months after an infection and can reappear during a recurrent infection, the results are often inconclusive on their own.
To determine the approximate timing of the infection, an IgG avidity test is performed. Low IgG avidity indicates that the antibodies are “new” and strongly suggests a primary infection occurred within the last three to four months, which carries the highest risk. Conversely, high avidity essentially rules out a primary infection in the current pregnancy, indicating a much lower risk of severe fetal consequences.
If a recent primary maternal infection is confirmed, fetal infection can be investigated using amniocentesis. This invasive procedure tests the amniotic fluid for CMV DNA using Polymerase Chain Reaction (PCR). The procedure is typically performed after 21 weeks of gestation and at least six weeks after the suspected maternal infection to ensure accuracy. A positive result indicates fetal infection, and dedicated fetal ultrasounds are used to look for physical signs of injury, such as brain or liver abnormalities.
For newborns, the definitive diagnosis of congenital CMV infection must be made within the first 21 days of life to distinguish it from a non-congenital infection acquired after birth. The most common method involves a PCR test on a saliva swab or urine sample to detect the virus’s DNA. Saliva testing is often used for initial screening, but a positive result is generally confirmed with a urine PCR test, which is considered the gold standard for post-natal diagnosis.