Fetuses are rarely infected with COVID-19 because the placenta produces very small amounts of the receptor that the coronavirus latches on to when it invades human cells, according to a study.
The placenta plays a role in stopping harmful substances from being passed down from a mother to her unborn child -- but certain pathogens like the Zika virus are known to cross this barrier frequently.
The new study, which was led by Roberto Romero at the US National Institutes of Health, and published in the journal eLife on Tuesday, sequenced genetic material taken from placental membranes that contain the fetus and amniotic fluid.
The researchers found it lacked the instructions required to manufacture a protein called angiotensin-converting enzyme 2 or, ACE2, which is found throughout the body in adults.
The SARS-CoV-2 virus is thought to enter the human body through its airways, then attack organs with high levels of ACE2 receptors, including but not limited to the lungs, blood vessels, kidneys and digestive system.
The placenta's genetic material also lacked the instructions to make an enzyme, called TMPRSS2, that SARS-CoV-2 uses to enter a cell.
"The molecules that are required to make the cells susceptible to SARS-CoV-2 infection are rarely expressed in the placenta," Romero told AFP.
To cross-check their methodology, the team then examined whether the genetic instructions used to create the cell receptors attacked by the Zika virus and cytomegalovirus were present, and found these were abundantly expressed.
"This is a way to be sure that the experiments we are doing are meaningful," added Romero.
So-called "vertical transmission" of the coronavirus from a pregnant mother to her fetus has been the subject of intense interest since the start of the pandemic.
On Tuesday, doctors in France said they had confirmed a case where this occurred in a baby boy born in March with brain swelling and neurological symptoms from which he later recovered.
But the overall prevalence of such transmission is thought to be extremely rare -- on the order of two percent or less.
Romero's team wrote in their paper that SARS-CoV-2 might infect fetuses by interacting with other proteins, not the ones they looked for -- but further research was needed.