Mothers can pass coronavirus to their babies in the womb and the virus may cause brain inflammation in newborns, a new study has suggested.
Although there have been several cases of babies contracting coronavirus, it was not known whether they were picking it up while inside the womb or catching it from close contact after birth.
However, on Tuesday doctors at Paris-Saclay University Hospital published a report describing a case in which coronavirus was found in the placental tissue of a 23-year-old mother whose baby was diagnosed with the virus after birth.
The specialists said it was "likely" that transmission had occurred when the baby was still in the womb, through the placenta.
"These findings confirm that transplacental transmission is indeed possible in the last weeks of pregnancy," they wrote in the journal Nature Medicine.
"The viral load is much higher in the placental tissue than in amniotic fluid or maternal blood: this suggests the presence of the virus in placental cells."
The baby was born early by Caesarean section because of concerns about his heart rate racing prior to birth, which may have been caused because his mother had a high fever caused by Covid-19.
The mother was discharged six days after the birth, but the baby was transferred to intensive care. Scans later showed that there was similar inflammation in the brain to that usually seen in adults.
The baby was discharged after 18 days, and doctors said his brain had started to recover after two months of follow-up.
Commenting on the report, Christoph Lees, professor of obstetrics at Imperial College London, said: "This is the most convincing case yet reported of transplacental transmission.
"We now have data from several thousand of pregnancies where mothers had the virus. In some cases, the babies are likely to have caught the infection either transplacentally, in the process of labour/delivery or afterwards.
"The great majority of women who test positive for SARS-CoV-2, or indeed have Covid-19, have normal pregnancy outcomes with healthy babies."
There have been 244 live born babies of mothers infected with coronavirus in Britain, and 95 per cent showed no sign of the virus.
Marian Knight, professor of maternal and child population health at the University of Oxford, said: "The most important message for pregnant women remains to avoid infection through paying attention to hand-washing and social distancing measures."