Cytomegalovirus (CMV) is an extremely common virus belonging to the herpes family; once infected, the virus remains in the body for life. For most healthy adults and children, CMV causes no symptoms or only mild, flu-like illness. It becomes a health concern primarily for individuals with weakened immune systems and, most notably, for pregnant individuals. When acquired during pregnancy, the virus can be passed to the developing fetus, potentially leading to serious and lasting health issues for the baby.
Understanding CMV and Pregnancy Risk
Over half of adults in the United States have been infected by age 40, and global seroprevalence approaches 83%. This widespread prevalence means many women of childbearing age already possess immunity to the virus. The risk of congenital infection is highest for the roughly 50% of U.S. women who have not been previously exposed and are susceptible to a primary infection during pregnancy.
A primary CMV infection occurs in approximately 1% to 2% of all U.S. pregnancies annually. When a pregnant person experiences this initial infection, the risk of transmitting the virus to the fetus is substantial, ranging from 30% to 50%. This risk is particularly elevated if the infection occurs during the first half of the pregnancy.
A non-primary infection—a reactivation of the latent virus or reinfection with a different strain—also poses a small risk of transmission. Non-primary infections occur in up to 10% of pregnancies among women who are already seropositive. The risk of fetal transmission during a non-primary infection is significantly lower than with a primary infection, typically falling between 0.5% and 2%.
The overall result of these transmission rates is that about one in every 200 babies in the U.S. is born with congenital CMV infection. This makes CMV the most common infectious cause of birth defects.
How CMV Spreads and Simple Prevention Steps
CMV is transmitted through direct contact with infected bodily fluids, including saliva, urine, tears, blood, semen, and breast milk. For pregnant individuals, the most frequent source of infection is exposure to the bodily fluids of young children. Toddlers often shed the virus in their saliva and urine for months after their initial infection. Individuals who work in childcare or have young children at home are therefore at a higher risk of acquiring the virus during pregnancy.
Preventative hygiene measures are the only current method to reduce the risk of maternal infection, as no vaccine is yet available. Handwashing is the most effective defense. This involves scrubbing with soap and water for 15 to 20 seconds, especially after changing diapers, feeding a young child, or wiping a child’s nose or drool.
Additional steps focus on minimizing direct contact with fluids that could contain the virus.
Minimizing Contact Risk
Pregnant individuals should avoid sharing food, drinks, or eating utensils with young children. They should also refrain from sharing toothbrushes or putting a child’s pacifier into their own mouth. Instead of kissing a child directly on the lips, a kiss on the forehead or a hug reduces contact with saliva. Frequently cleaning toys, countertops, and other surfaces that may have come into contact with a child’s urine or saliva also helps limit the virus’s spread.
Outcomes of Congenital CMV Infection
When CMV is passed to the developing baby, the consequences vary widely, ranging from no apparent issues to severe, life-long disabilities. The vast majority of infants with congenital CMV (approximately 85% to 90%) appear healthy and asymptomatic at birth. However, 10% to 15% of these initially asymptomatic infants will develop long-term problems that may not be noticeable until years later.
For the 10% to 15% of babies who are symptomatic at birth, the risk of permanent sequelae is much higher, affecting 50% to 70% of this group. Initial symptoms can include microcephaly, jaundice, rash, an enlarged liver or spleen, and low birth weight.
The most common long-term disability resulting from congenital CMV is sensorineural hearing loss (SNHL). SNHL can affect one or both ears and may be present at birth or progress over time, often requiring ongoing monitoring.
Beyond hearing loss, the virus can lead to a spectrum of neurodevelopmental impairments. These severe outcomes include developmental delays, intellectual disability, cerebral palsy, vision impairment due to chorioretinitis, and epilepsy. The severity of these outcomes is linked to the trimester in which the mother acquired the primary infection, with first-trimester infections posing the highest risk of severe neurological damage.
Testing and Treatment Options
Diagnosis typically begins with maternal serology, measuring Immunoglobulin G (IgG) and Immunoglobulin M (IgM) antibodies in the blood. If both IgM (recent infection) and IgG (exposure) are present, an IgG avidity test is performed to determine the approximate timing. Low avidity indicates a recent primary infection, while high avidity suggests a past infection.
If a primary maternal infection is confirmed, fetal testing involves an amniocentesis to analyze amniotic fluid for CMV DNA using a polymerase chain reaction (PCR) test. This procedure is performed at least eight weeks after the estimated infection time and after 21 weeks of gestation to maximize accuracy.
Congenital CMV infection is confirmed after birth by detecting the virus’s DNA in the newborn’s urine, saliva, or blood via a PCR test conducted within the first 21 days of life.
There is no treatment to cure the infection in the mother or prevent fetal transmission. For newborns diagnosed with symptomatic congenital CMV, antiviral treatment can improve long-term outcomes. The standard medication is oral Valganciclovir, administered for six months. Starting treatment within the first four weeks of life helps prevent the progression of hearing loss and improves neurodevelopmental results.