London: An unborn baby could become infected with COVID-19 if their gut is exposed to the SARS-CoV-2 virus, claims a new study.
The study led by University College London researchers found that certain foetal organs, such as the intestine, are more susceptible to infection than others.
However, researchers said that opportunities for the COVID-19 virus infecting the foetus are extremely limited, as the placenta acts as a highly effective and protective shield, and evidence suggests foetal infection, known as vertical transmission, is extremely uncommon.
"Our findings explain that clinical infection of the foetus during pregnancy is possible but uncommon and that is reassuring for parents-to-be," said Dr Mattia Gerli, from UCL's Division of Surgery and Interventional Science and the Royal Free Hospital.
For the study, published in BJOG - An International Journal of Obstetrics & Gynaecology, researchers set out to understand how newborn babies could have developed COVID-19 antibodies, as has been reported in a small number of cases and how the virus could be passed from an infected mother to the unborn foetus.
To find out, researchers examined various foetal organs and placenta tissue, made available via the Human Developmental Biology Resource (HDBR) biobank, which assists embryonic/fetal research.
The team examined the presence of protein receptors, ACE2 and TMPRSS2 -- needed for the SARS-Cov-2 virus to infect and spread.
Researchers found that the intestines (gut) and the kidney were the only foetal organs to feature both the ACE2 and TMPRSS2.
Since the foetal kidney is anatomically protected from exposure to the virus and therefore less at risk of infection, the team concluded that the COVID virus could infect the foetus via the gut and through foetal swallowing of amniotic fluid, which the unborn baby does naturally for nutrients.
After birth ACE2 and TMPRSS2 receptors are known to be present in combination on the surface of cells in the human intestine as well as the lung -- also suspected to be the main routes for COVID infection. But in younger children, the intestine appears to be most important for virus infection.
"The foetus is known to begin swallowing the amniotic fluid in the second half of pregnancy. To cause infection, the SARS-CoV-2 virus would need to be present in significant quantities in the amniotic fluid around the foetus," Gerli explained.
"However, many studies in maternity care have found that the amniotic fluid around the foetus does not usually contain the SARS-CoV2 virus, even if the mother is infected with COVID-19," she noted.
The researchers highlighted that the biggest risk to the foetus during pregnancy is if the mother becomes very unwell with COVID-19 infection. In this instance the virus may be present in high concentration in the amniotic fluid. In addition, it could damage the placenta, which can lead to preterm birth.
"Vaccination against COVID-19 is known to be safe in pregnancy and reduces the chance of SARS-CoV-2 infection to very low levels. The results of this study provide definitive information regarding the susceptibility of the human foetus to COVID-19 infection," said co-author Professor Anna David from UCL Elizabeth Garrett Anderson Institute for Women's Health.