TORONTO -- A baby girl in Texas tested positive for COVID-19 shortly after her birth, offering what researchers call the strongest evidence yet that the disease can be transmitted from mothers to children in the womb.

There have been several well-documented cases of female COVID-19 patients giving birth while positive for the disease. Doctors were even able to deliver a baby boy in Colombia last month 14 weeks early, after the mother slipped into a coma.

Researchers have also found evidence of the novel coronavirus in at-term placenta, an umbilical cord and breast milk of infected mothers.

However, there had been no known examples of a baby born to a COVID-19-positive mother also testing positive for the disease – not until Friday, when researchers affiliated with the University of Texas published their findings in The Pediatric Infectious Disease Journal.

In this case, the baby was born prematurely, at 34 weeks. Because of that and because of the possible exposure to COVID-19 through her 37-year-old mother, she was placed in a neonatal intensive care unit for observation.

Although all her vital signs were normal on the day of her birth, the researchers say, she developed a fever and some mild breathing difficulty the following day. A test administered 24 hours after her birth came up positive for SARS-CoV-2, as did a second test 24 hours later.

Further tests of the placenta revealed the presence of coronavirus particles and a protein that is believed to be specific to the virus. Because of these results, the researchers concluded that COVID-19 was passed on to the baby in the womb, not during or after her birth.

The researchers say this sort of transmission "appears to be a rare event" given that no other documented cases of it have come to light.

The baby was given supplemental oxygen for a few days and continued to test positive through her first two weeks of life. Three weeks after she was born, she and her mother were sent home, and both are said to be in good condition.