A prenatal infection is one that a baby acquires before or during birth, often passed from the birthing parent. These infections are a serious health concern because the developing fetus is highly susceptible to damage from invading pathogens. Once a pathogen crosses the placental barrier, it can cause significant and lasting injury to growing organ systems. Understanding the most frequent cause of these infections is the first step in addressing the risk to infant health and development.
The Most Frequently Encountered Prenatal Infection
The most common congenital infection worldwide is Cytomegalovirus (CMV), a member of the herpesvirus family. Like other herpesviruses, CMV remains in the body for life after the initial infection, often lying dormant. Over half of adults have been infected with CMV by age 40, though most are unaware they contracted it.
CMV is the leading infectious cause of birth defects in the United States, affecting approximately one in every 200 babies born. This makes it more common than other congenital conditions like Toxoplasmosis or Rubella. Although the majority of infected newborns will not show signs of illness at birth, congenital CMV is the primary non-hereditary cause of sensorineural hearing loss in children.
Transmission Routes and Maternal Presentation
The CMV virus is transmitted through direct contact with body fluids, including saliva, urine, blood, tears, semen, and breast milk. Young children are the most frequent source of transmission to pregnant individuals, as they often shed the virus in high concentrations in their saliva and urine for months after infection. Exposure often occurs in household or daycare settings, such as during diaper changes or when sharing food.
The infection in a pregnant individual is often silent, with about 90% experiencing no noticeable symptoms. When symptoms occur, they are typically mild and non-specific, resembling a common cold or mononucleosis, with possible fever, fatigue, and a sore throat. This lack of distinct symptoms means the infection often goes undiagnosed during pregnancy, complicating efforts to manage the risk to the fetus.
The risk of passing the virus to the fetus is highest during a primary infection—the first lifetime exposure to CMV—during pregnancy. In these cases, the transmission rate can be as high as 30% to 40%. While a recurrent infection (reactivation or a new strain) can also be transmitted, it carries a substantially lower risk of severe fetal complications.
Health Implications for the Infant
The consequences of congenital CMV infection range widely, from no long-term effects to severe, permanent disabilities. Approximately 85% to 90% of babies with congenital CMV are asymptomatic at birth. However, up to 15% of those infants can still develop long-term health problems, most commonly progressive hearing loss.
The most frequent long-term disability is sensorineural hearing loss (SNHL). This hearing loss can be present at birth or develop months to years later, affecting one or both ears and potentially worsening over time. This underscores the need for regular audiologic monitoring. For the smaller percentage of infants who are symptomatic at birth, the potential for severe health issues is much greater.
These symptomatic infants may exhibit signs like jaundice, rash, low birth weight, an enlarged liver and spleen (hepatosplenomegaly), and a small head size (microcephaly). The virus can damage the central nervous system, leading to intellectual disability, developmental delays, vision problems, and coordination issues. While only a small fraction of infected newborns are symptomatic at birth, these babies face a higher risk of mortality and severe, lifelong neurological impairment.
Reducing Risk and Management Options
Since there is currently no vaccine available for CMV, prevention focuses on modifying behaviors to reduce exposure to the body fluids of young children. Simple, consistent hygiene practices are the most effective way to lower the risk of infection, especially for pregnant individuals who care for small children. Washing hands frequently and thoroughly with soap and water for at least 20 seconds is recommended after changing diapers, feeding a child, or handling children’s toys.
Pregnant people should avoid putting a child’s pacifier in their own mouth and should not share food, drinks, or eating utensils with young children. Caregivers should kiss young children on the forehead or give a hug, instead of kissing them directly on the mouth. These steps are relevant because the virus can be shed in a child’s urine and saliva for an extended period.
For infants diagnosed with congenital CMV, early detection is important for improving long-term outcomes. Newborns who show symptoms of the infection may be treated with an antiviral medication, most commonly oral valganciclovir. A typical course of treatment lasts for six months and has been shown to improve hearing and neurodevelopmental outcomes. Even for babies who are asymptomatic at birth, regular hearing checks are necessary for several years to catch late-onset hearing loss.