A prenatal infection is a disease acquired by the fetus inside the uterus, typically passed from the mother through the placenta. These infections are a serious public health concern due to their potential to cause developmental delays and long-term disability. The most common infectious agent transmitted during pregnancy is Cytomegalovirus (CMV), which affects approximately one in every 200 babies born in the United States annually. CMV is the most common infectious cause of birth defects and nonhereditary sensorineural hearing loss.
Identifying the Most Common Threat
Cytomegalovirus is a highly prevalent virus belonging to the herpesvirus family. Once a person is infected, the virus remains in the body for life, often in a latent state. More than half of adults in the United States have been infected with CMV by age 40, and most healthy people experience no symptoms or only a mild, flu-like illness. This widespread presence makes it the leading cause of congenital viral infection globally.
The risk to a developing fetus depends heavily on the timing and type of maternal infection. A primary infection, which is a mother’s first exposure to the virus, poses the highest danger. Non-primary infections, involving reactivation of the latent virus or exposure to a different strain, can also be transmitted. Primary infection during pregnancy carries a transmission risk of about 30% to 40% to the fetus.
Modes of Transmission During Pregnancy
Vertical transmission occurs when the virus crosses the placental barrier to reach the developing fetus. This transfer can happen at any point during gestation, and the rate of transmission increases as the pregnancy progresses. While the risk is relatively low in the first trimester, it can rise to as high as 70% in the third trimester following a primary maternal infection.
The severity of the resulting fetal injury is inversely related to the timing of the infection, meaning infections acquired earlier in pregnancy are more likely to cause severe complications. The mother’s body fluids, such as saliva and urine, are the main sources of viral spread. Contact with young children is a major risk factor for pregnant women who have not previously been infected. The virus can also be transmitted during delivery or after birth through breast milk, but these routes typically do not cause the severe central nervous system damage seen with congenital infection.
Recognising Maternal and Fetal Symptoms
A significant challenge in managing CMV is the silent nature of the maternal infection. Approximately 80% to 95% of pregnant women who acquire a primary infection will not exhibit any symptoms, making diagnosis difficult. When symptoms do occur, they are often nonspecific, resembling mononucleosis or a mild cold, with signs like fever, fatigue, or swollen glands.
Fetal involvement can sometimes be suggested by specific findings on a prenatal ultrasound, such as intrauterine growth restriction or microcephaly. Once the baby is born, only about 10% of those with congenital CMV show signs of illness at birth. These symptomatic newborns may present with jaundice, low birth weight, an enlarged liver and spleen (hepatosplenomegaly), or a petechial rash. Severe cases can also include seizures and damage to the retina, known as retinitis.
Health Outcomes for the Newborn
Congenital CMV is the most frequent infectious cause of long-term neurodevelopmental disabilities in children. Outcomes are generally divided between infants who are symptomatic at birth and those who are asymptomatic. Symptomatic infants face a much higher risk of severe disability and mortality, with over 90% likely to experience serious complications such as intellectual disability, vision problems, and hearing loss.
The most common long-term consequence for all affected infants is sensorineural hearing loss (SNHL). This hearing loss can be present at birth or may develop later in childhood, even in babies who were initially asymptomatic. About 10% to 15% of infants who appear healthy at birth will eventually develop delayed sequelae, with SNHL being the most frequent concern. Other lasting effects can include developmental delay, vision loss, and issues with coordination.
While there is currently no licensed vaccine to prevent CMV, good hygiene practices are recommended for pregnant women, particularly those exposed to young children. Antiviral medications like valganciclovir are available to treat symptomatic newborns, which can potentially improve long-term outcomes.