Vertical Transmission: How Moms With COVID-19 Might Pass Coronavirus to Babies

Experts say it's probably a "pretty rare" occurrence, but it's not unheard of.

These are unsettling times for moms-to-be and women who are planning to conceive. As the number of novel coronavirus cases continues to spike, women in the US and around the globe inevitably worry about becoming infected themselves and unwittingly transmitting the virus to their newborns.

Previously, a Lancet study published in February involving nine pregnant women in China who had COVID-19 pneumonia found no evidence that their babies acquired the virus. That was comforting news, but now a new study in JAMA Pediatrics suggests that "vertical transmission"—from mom to baby—of the virus that causes COVID-19, may be possible. And separately, two case reports in JAMA (the Journal of the American Medical Association) question whether the virus might even be transferred in utero.

While the new reports are troubling to think about, women ought to take comfort in the fact that all of the babies in these studies born to mothers with COVID-19 survived, and the ones who had symptoms fully recovered from their illness.

What's more, the US Centers for Disease Control and Prevention (CDC) now advises healthcare providers that transmission of the virus to newborns is thought to occur primarily from respiratory droplets when babies are exposed to moms, caregivers, visitors, or healthcare workers who are infected. It concedes, though, that "the extent and clinical significance of vertical transmission" is unclear.

Here's what the latest science tells us.

What is vertical transmission?

Vertical transmission is defined as mother-to-child transmission of a pathogen (such as a virus, like the coronavirus or SARS-CoV-2). It can be passed along during pregnancy (in the uterus), during labor or delivery (say, when the baby descends through the birth canal), or immediately afterward (via breastfeeding, for example).

Doctors use a variety of terms to describe these mom-to-fetus or mom-to-infant transmissions:

  • A congenital infection means the fetus or baby acquired the illness during pregnancy (in utero) or at delivery, according to the Emory University School of Medicine.
  • A perinatal infection is one that occurs at or around the time of delivery, per the Pan American Health Organization.
  • Intrapartum transmission occurs when an infant is exposed to an infected mom's vaginal secretions or blood during labor or delivery.
  • In utero transmission occurs when a virus (or bacteria or some other pathogen) passes through the placenta to the fetus.

It's worth noting that CDC recently issued interim guidance on breastfeeding in light of the COVID-19 pandemic. "We do not know whether mothers with COVID-19 can transmit the virus via breast milk," says CDC, "but the limited data available suggest this is not likely to be a source of transmission."

Can a baby really contract COVID-19 from the mom?

While there's no definitive proof, the latest evidence suggests that vertical transmission may be possible. The study in JAMA Pediatrics involved 33 babies born to mothers in China with COVID-19. Overall, the infants' symptoms were generally mild, and their outcomes were good. The most common symptom (exhibited by four of the 33 babies) was shortness of breath.

However, three of the newborns tested positive for the virus. One became lethargic and spiked a fever the day after delivery, according to researchers. A second infant was lethargic and had fever and vomiting, they reported. The two babies (delivered full term by C-section) also had chest x-rays showing pneumonia. Testing performed on the babies' second and fourth days of life came back positive for SARS-CoV-2, the virus that causes COVID-19. By day six, though, nasal and anal swab samples came back negative for the virus.

A third, very sick infant, who was delivered prematurely by C-section, had to be resuscitated. The child was treated for neonatal respiratory distress syndrome, pneumonia, and suspected sepsis. Swab testing for the virus came back positive on days two and four—and negative on day seven.

Still, it's hard to say for sure whether the babies actually contracted COVID-19 from their mothers, according to experts. While the report's authors say the hospital practiced strict infection control and prevention during delivery, it's still possible that the babies picked up the virus in the nursery, David Kimberlin, MD, a pediatric infectious disease specialist at the University of Alabama at Birmingham (UAB), tells Health. But, he adds, the timing of the detection, coupled with the fact that the mothers had confirmed cases of COVID-19, "suggests that there was perinatal transmission."

Dr. Kimberlin suspects it's "an unusual occurrence" since there haven't been widespread reports of mom-to-baby COVID-19 transmission since the virus was first detected in China in December 2019. If other cases existed, we would have heard about them by now, he reasons, "so I'm speculating that it's a pretty rare kind of thing."

It should be noted that the affected babies had PCR testing. That's short for polymerase chain reaction, a type of testing that confirms whether the virus' genetic material is present. So, unless there was a lab error or a "body error" (meaning the babies were so sick that their test results were not reliable), PCR test results that convert to negative from positive indicate that the babies cleared the virus from their bodies, Rahul Gupta, MD, senior vice president and chief medical and health officer at the March of Dimes, tells Health.

Dr. Kimberlin says the fact that all three infants cleared the virus within a week is encouraging. "For [those] three babies, that's good news, and I think we have to celebrate that," he says.

Case reports from China, published online in JAMA, also suggest that COVID-19 might be transferred in utero. But experts say that these findings aren't terribly convincing. That's because the reports rely on IgM testing—short for immunoglobulins testing—which measures antibodies to the virus, not the actual virus.

One report described an infant girl with elevated IgM antibody levels. Another report on six infants found that two of them had higher-than-normal IgM levels. Although the women in the reports had COVID-19, none of their babies tested positive for the virus. Nor did the newborns exhibit symptoms.

In a JAMA editorial accompanying the reports, Dr. Kimberlin and his UAB colleague Sergio Stagno, MD, pointed out that IgM isn't very reliable in detecting congenital infections because of false-positives, false negatives, and other testing issues. "It raises the possibility of in utero acquisition," Dr. Kimberlin tells Health. "But it could also be a false-positive test or some sort of contamination or some sort of cross-reactivity with another antibody. It wasn't really the SARS-CoV-2 antibody that was being detected." Dr. Gupta adds that "IgM is a larger molecule; it's much harder to transmit through the placenta in utero."

The bottom line here: It's way too early to know whether IgM levels are evidence that a fetus may contract the virus in the womb.

How can pregnant women protect themselves and their babies?

So far, most of the data on mom-to-baby transmissions involve women close to or in their third trimester, notes Dr. Gupta. It's unclear what happens to babies when their moms acquire COVID-19 in the first or second trimester.

If you're pregnant and suspect you have the infection, contact your healthcare provider for specific advice on managing your condition, advises Dr. Gutpa.

Keep in mind, though, that you can be infected without even knowing it. Between March 22 and April 4, clinicians at Columbia University Irving Medical Center in New York City screened 215 pregnant women who were admitted for delivery for symptoms of COVID-19. Only four presented with symptoms, and each of them tested positive. Of the remaining 211 women, 210 were tested for the coronavirus. Of that asymptomatic group, 29 of them, or 13.7%, tested positive for SARS-CoV-2, according to an April 13 letter to the New England Journal of Medicine. The report did not mention the status of their newborns.

For now, the advice for pregnant women is the same as for other high-risk groups.

"It's important for women, first and foremost, to ensure that they are doing everything they can to prevent getting COVID-19," says Dr. Gupta. That means following infection control practices, including handwashing and social distancing—but it doesn't mean isolating yourself in the house. You still need to exercise, eat properly, and go to your prenatal appointments, he says.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDC, WHO, and their local public health department as resources.

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