Evidence has emerged that COVID-19 infection might be linked to brain haemorrhages in unborn children.
Scientists who examined tissues from foetuses said it was "extremely unusual" for the number of brain haemorrhages detected to have occurred.
The cause of these haemorrhages is unclear.
Possible explanations might be they happened as a direct consequence of the infection or an indirect consequence of the maternal immune response, the King's College London researchers said.
Their study suggests that COVID might affect the foetal brain during the earliest stages of gestation, highlighting a need for further research into the potential impact on subsequent neurological development.
The scientists studied 26 samples of human foetal tissue with observed haemorrhages from a total of 661 samples collected between July 2020 and April 2022.
Coronavirus was present in all of the samples with haemorrhages.
Dr Katie Long, the study’s lead investigator from the Institute of Psychiatry, Psychology & Neuroscience at King's, said: "While haemorrhages do occasionally occur in developing brains, it is extremely unusual for there to be this many instances within a 21-month period.
"It is now of the utmost importance that we follow up with children that were prenatally exposed to COVID-19 so that we can establish if there are any long-lasting neuro-developmental effects.
"While we haven't yet been able to establish clear causation, we certainly feel that it highlights a need for expectant mothers to take extra precautions at a time when cases are on the rise."
Marco Massimo, the study's first author, said: "Our findings suggest that there is an association between the early development of human foetal brain tissue and vulnerability to infection from COVID-19."
Professor Lucilla Poston, professor of maternal and foetal health at King's, said: "We know that severe viral infection may influence the foetal brain, but this important study is the first to suggest that this may occur in pregnancies affected by COVID infection.
"Whatever the cause, a direct effect of the virus or an indirect consequence of maternal infection, this study highlights the need for pregnant women to be vaccinated against COVID-19, thus avoiding complications for both mother and baby."
The majority of the haemorrhagic samples came from donated foetal tissue between the late first and early second trimester of gestation.
This is a particularly important period of human foetal brain development during which the tight junctions between endothelial cells of the blood vessels increase to form the blood brain barrier, the semi-permeable barrier that protects the brain from foreign substances.
Upon further study, the integrity of the blood vessels within the haemorrhagic samples was found to be considerably lower than the non-infected samples.
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