Viral Transmission in the Womb: Zika and Coronavirus

Dive Deeper

Why do some viruses, like Zika virus, infect the unborn while others, like COVID-19, rarely do? The answer lies in the placenta, the connection between the mother and the fetus. Viruses use passageways into the cell called receptors. The coronavirus mostly uses two receptors called ACE2 and TMPRSS2 to get inside of cells.1 The placenta has negligible numbers of these receptors.2 Therefore, it is very difficult, though not impossible,3 for the fetus to get infected with COVID-19 while in the womb of an infected mother. Nevertheless, COVID-19 can cause inflammation in the placenta, which could explain why the virus increases the risk of preterm birth, premature rupture of membranes, and miscarriage.4

On the other hand, the Zika virus uses different receptors to pass from the mother to the fetus. These receptors are much more common in the placenta.5 Therefore, more fetuses get Zika virus infections in the womb. Furthermore, the Zika virus stops cells from becoming brain cells, and destroys developing brain cells causing the newborn to have a smaller brain.6 The earlier in the pregnancy that the mother gets infected, the fewer healthy brain cells are already in the correct locations in the fetal brain, and the more likely that the infant will have a neurological birth defect.7

This CT scan of a pregnant mother shows the placenta slightly above the fetus near the mother's spine. The placenta is the main connection between the mother and the fetus. Some infections, like the Zika Virus, transfer from the mother to the baby while most bacterial infections don’t. The placenta guards the baby from most outside infections, but has receptors that let some infectious organisms in. (Image Credit: Mikael Häggström, 26 July 2017)