New York, Feb 14 (IANS): Covid-19 infection is known to cause stillbirths in unvaccinated pregnant women, but the mechanism was not yet understood.

A 44-member international research team studied 64 stillbirth cases and four early neonatal deaths from 12 countries to determine how Covid-19 caused perinatal deaths in unvaccinated expecting mothers, USA Today reported.

The findings, published in Archives of Pathology & Laboratory Medicine, showed that the Covid-19 infection destroys the placenta, depriving the foetus of oxygen.

Researchers determined the virus reaches the placenta and causes it to fail by passing through the mother's bloodstream, a process known as viremia.

"Our study identified placental insufficiency as the root cause for stillbirths in women with Covid-19 during pregnancy," said Dr David Schwartz, an Atlanta-based pathologist who led the study, was quoted as saying.

"Among the 68 cases, an average of 77 per cent of the placenta had been destroyed and rendered useless for supporting critical foetal needs, resulting in stillbirth or early neonatal death," he added.

In all the studied cases, researchers found the placentas from infected mothers had a severe abnormality called SARS-CoV-2 placentitis, Schwartz said.

The team also found viral-induced lesions in the placenta blocked maternal and foetal blood flow and oxygen, killing placental tissues and causing "irreparable damage", the report said.

Further, in almost all the cases, an increase was observed in fibrin -- a key protein involved with blood clotting -- was so "massive" it blocked blood and oxygen flow to the placenta.

All the placentas also showed dead cells made up the major cell barrier between the mother and foetus, known as trophoblast necrosis.

Another placental complication that may have been caused by the virus was a rare accumulation of inflammatory cells called chronic histiocytic intervillositis, which was seen in 97 per cent of cases studied by the international research team.

Moreover, the researchers assumed that many of the infections were from the Delta variant, and not Omicron.

Although other viral, bacterial and parasitic infections that occur in pregnancy and cause stillbirth travel through the placenta and damage the foetal organs, but Schwartz said SARS-CoV-2 appears to stop at the placenta and do the most damage there.

"The placental destruction is so severe that whether or not the foetus becomes infected might be irrelevant," he said.

Studies have shown that the Covid vaccine is safe and effective for both the expectant parent and baby. The studies have also shown that antibodies from the vaccine can pass through to the foetus and protect the baby from Covid-19 after birth.