The emergence of SARS-CoV-2, the virus responsible for COVID-19, introduced many unknowns for pregnant individuals and their developing fetuses. A primary concern centered on the potential for the virus to pass from the mother to the baby during gestation. This process, known as vertical transmission, occurs when a pathogen in the mother’s bloodstream crosses the placental barrier to infect the fetus. Understanding the likelihood and mechanism of this transmission is important for guiding clinical care and providing accurate information.
Establishing Vertical Transmission
The question of whether SARS-CoV-2 crosses the placenta has a nuanced answer: it is possible, but infrequent. True vertical transmission has been confirmed in only a small percentage of cases involving maternal COVID-19 infection. Studies consistently show that the rate of SARS-CoV-2 positivity in newborns is low, estimated to be in the range of 2.66% to 3.2% of infants born to infected mothers.
This low rate is significant compared to viruses like Rubella or Zika, which have a much higher rate of transplacental passage. Confirming true in utero infection is challenging because it must be distinguished from infection acquired during or immediately after birth. Definitive confirmation requires detecting the viral genetic material in samples secured before or immediately at delivery, such as umbilical cord blood or amniotic fluid. Even with strict criteria, the consensus is that the virus breaches the barrier with much less regularity than other known congenital pathogens.
The Placental Mechanism of Infection
The placenta is a highly specialized organ that functions as a physical and immunological barrier between the maternal and fetal blood supplies. This barrier is maintained by syncytiotrophoblasts, a layer of cells in direct contact with the mother’s blood. For SARS-CoV-2 to infect the fetus, it must first successfully infect these cells and then propagate toward the fetal circulation.
The virus initiates infection by binding to the Angiotensin-Converting Enzyme 2 (ACE2) receptor on the host cell surface. Researchers confirmed that this ACE2 receptor is expressed on the syncytiotrophoblasts, supporting the plausibility of placental infection. When vertical transmission occurs, it is often associated with a distinct pathology called “placentitis,” which involves inflammation and damage within the placental tissue. This damage suggests the virus may compromise the integrity of the syncytiotrophoblast layer, creating a pathway for viral entry.
Fetal and Neonatal Outcomes
While the rate of the virus crossing the placenta is low, the consequences of maternal COVID-19 infection on fetal and neonatal well-being are substantial, primarily due to indirect risks. Adverse outcomes are frequently linked to the severity of the mother’s illness and resulting systemic inflammation, rather than direct fetal infection. Severe maternal infection is associated with an elevated risk of preterm birth (delivery before 37 weeks of gestation).
Severe maternal COVID-19 also increases the likelihood of complications, including pre-eclampsia, stillbirth, and a higher rate of medically indicated cesarean delivery. Neonates born to mothers with severe infection are more likely to require admission to a neonatal intensive care unit (NICU). In rare instances of confirmed in utero infection, most newborns experience no symptoms or only mild illness. However, more severe outcomes, including Multisystem Inflammatory Syndrome in Children (MIS-C) or in neonates (MIS-N), have been documented weeks after the initial infection.
Clinical Recommendations for Expectant Mothers
The best defense for both the mother and the developing baby remains prevention against severe COVID-19 illness. Major medical bodies recommend that pregnant individuals receive the updated COVID-19 vaccine at any point during their pregnancy. Vaccination is a safe and effective strategy that reduces the risk of severe disease, hospitalization, and death for the mother.
The benefits of maternal immunization extend to the infant through the transplacental passage of protective antibodies. These antibodies provide the newborn with immunity against severe disease in the months following birth. Expectant mothers should also follow general public health measures, such as frequent hand washing and considering masks in crowded indoor settings. If symptoms develop, contact an obstetrician immediately for guidance on testing and management to ensure close monitoring of maternal and fetal status.