A baby born in a Paris hospital in March to a mother with Covid-19 tested positive for the virus and developed symptoms of inflammation in his brain, said Dr. Daniele De Luca, who led the research team and is chief of the division of pediatrics and neonatal critical care at Paris-Saclay University Hospitals. The baby, now more than 3 months old, recovered without treatment and is “very much improved, almost clinically normal,” Dr. De Luca said, adding that the mother, who needed oxygen during the delivery, is healthy.

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Dr. De Luca said the virus appeared to have been transmitted through the placenta of the 23-year-old mother.

Since the pandemic began, there have been isolated cases of newborns who have tested positive for the coronavirus, but there has not been enough evidence to rule out the possibility that the infants became infected by the mother after they were born, experts said. A recently published case in Texas, of a newborn who tested positive for Covid-19 and had mild respiratory symptoms, provided more convincing evidence that transmission of the virus during pregnancy can occur.

In the Paris case, Dr. De Luca said, the team was able to test the placenta, amniotic fluid, cord blood, and the mother’s and baby’s blood.

The testing indicated that “the virus reaches the placenta and replicates there,” Dr. De Luca said. It can then be transmitted to a fetus, which “can get infected and have symptoms similar to adult Covid-19 patients.”