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.Still, Dr. Sadovsky said, it is important to note that cases of possible coronavirus transmission in utero appear to be extremely rare. It found that while cells in the placenta had many of the receptor proteins that allow viruses to propagate, there was evidence of only “negligible” amounts of a key cell surface receptor and an enzyme that are known to be involved in allowing the coronavirus to enter cells and replicate. The study was led by Dr. Robert Romero, chief of the perinatology research branch at the Eunice Kennedy Shriver National Institute of Child Health and Human Development. The report from doctors in Paris said that the woman was 35 weeks pregnant when she came to the hospital with a fever and a cough that she developed a couple of days earlier in what was an otherwise healthy pregnancy.