The study, published in the journal Nature Communications, described a case in France where the COVID-19-causing virus was found in the blood of a baby born prematurely to a 23-year-old mother who was diagnosed with the pandemic disease in March.
The detection of the virus in placental tissue, as well as in the mother's and baby's blood, suggests that "transplacental transmission of SARS-CoV-2 (the virus that causes COVID-19) may be possible," the doctors detailing the case wrote. They added, however, that further studies would be needed to confirm this.
Marian Knight, a professor of maternal and child population health at Britain's Oxford University, said the case was interesting, but should not be a major worry for pregnant women.
"Among the many thousands of babies born to mothers with SARS-CoV-2 infection, a very few have been reported to also have a positive test – around 1-2%," she said. "It is still unclear whether the virus passes across the placenta; this report provides evidence that it may."
Both mother and baby in the French case recovered well and were discharged from hospital.
Andrew Shennan, an obstetrics professor at King's College London, agreed it is rare for unborn babies to catch COVID-19 from their mothers. He cited UK data on 244 babies born to infected mothers, of which 95% had no sign of the virus."Women can remain reassured that pregnancy is not a significant risk factor for them or their babies with COVID-19," he said.