Pregnant women who have the coronavirus can possibly pass the infection onto their babies, though it's unclear whether this transmission occurs in the womb.
Although more research is needed, a small study, published in the journal JAMA Pediatrics on Thursday, found that of 33 women in China who were confirmed to have the coronavirus while pregnant, three gave birth to newborns who were then diagnosed with the virus. All three infants recovered, adding to a growing pile of evidence that most children experience milder cases of COVID-19, the illness caused by the coronavirus, than adults.
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Babies born to the other 30 women in the study tested negative for the virus.
Because the three newborns who tested positive were diagnosed so early in life — all three received positive tests on their second day of life and again on their fourth day of life — the source of the transmission was likely "maternal in origin," the study authors wrote.
What exactly that means is up for debate, said Dr. David Kimberlin, a professor of pediatrics at the University of Alabama at Birmingham and a pediatric infectious diseases physician, who was not involved in the study.
Very few viruses cross over into the placenta and infect babies in utero, Kimberlin said, although some — like Zika — do, with devastating effects. In the coronavirus study, the newborns' positive results, which were acquired through swab tests, "is more suggestive of the possibility of perinatal transmission around the time of delivery," he added.
The babies could have acquired the coronavirus from the birth canal or from microtears in their amniotic sacs, among other possibilities, he said.
But because the sample size is so small, more information is needed, according to Kimberlin, who co-authored an editorial about two other papers published Thursday in the Journal of the American Medical Association about whether the coronavirus can be acquired in utero.
Those papers suggested that infants born to infected mothers can have elevated antibodies for the virus detected in their own blood, although it is not clear whether the antibodies were made in response to the infection in the womb or simply made by the mother and transferred to the baby.
The data at this point are "not conclusive and do not prove in utero transmission," the editorial said.
The JAMA Pediatrics study offered encouraging news about infants' ability to recover from the coronavirus. Two of the three who were infected with it no longer tested positive by the sixth day of life, and the third, who had been born prematurely, was negative by the seventh day of life.
That infant had been born nearly nine weeks early due to complications the mother was experiencing from the coronavirus, including pneumonia. The infant needed more medical assistance to recover, although the researchers conceded his health problems may have been tied more to his prematurity than to the virus itself.
The other two sick babies were lethargic and had signs of pneumonia in their lungs, and one was vomiting during the weeklong course of their illness.
The premature infant was diagnosed with neonatal respiratory distress syndrome and pneumonia and needed to be resuscitated at birth. Two weeks into life, the respiratory symptoms associated with the coronavirus had resolved.
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Experts have a host of theories for why children generally seem to fare better from the coronavirus than adults do, including the size of the immune response that they launch to it, the size and maturity of a receptor in their lungs that the virus seems to attach to, and the possibility that school-age children might have cross-protection to this new coronavirus because they are constantly exposed to other viruses.
But no one fully understands why children are getting milder cases of it, including newborns.
"The honest answer is we don't know why that is," Kimberlin said. He cautioned that while the three babies in this study recovered well, "we need a lot more information before we can say that all babies are not at risk" for serious complications.
Regardless, experts do not recommend that pregnant women make any changes to their birth plans. The timing and delivery method should not be altered in most cases, according to the American College of Obstetricians and Gynecologists, and a hospital birth, not a home birth, is still the safest route.
"It’s much more likely there will be a problem with the delivery with a home birth than our current understanding of the likelihood of risk from a woman coming in to give birth in a hospital," Kimberlin said.
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