Their babies could potentially experience adverse outcomes, too, researchers found. They’re more likely to be born early and at a low birthweight—factors that could result in chronic health issues, feeding problems, a greater risk of sudden infant death syndrome (SIDS), and other complications later in life.

The study analyzed data from nearly more than 13,000 women who had experienced COVID during their pregnancies. The women hailed from 12 countries, including Ghana, China/Hong Kong, Italy, Kenya, Nigeria, South Africa, Spain, Sweden, the Democratic Republic of Congo, Turkey, Uganda, and the U.S. Most were unvaccinated, and their pregnancies generally occurred early in the pandemic.

Researchers found that women who had experienced COVID during their pregnancy were:

• nearly 8 times more likely to die.
• more than 23 times more likely to develop pneumonia.
• more than 15 times more likely to require ventilation.
• nearly 6 times more likely to experience thromboembolic disease.
• more than 5 times more likely to require critical care.
• nearly 4 times more likely to require ICU admission.
• slightly more likely to experience hypertensive disorder and/or pre-eclampsia, or to need a C-section.

Their babies were:

• more likely to be born early.
• slightly more likely to be born with a low birth weight (less than 5.5 pounds).

Dr. Mark Turrentine—an OBGYN and member of the American College of Obstetricians and Gynecologists’ COVID-19 expert working group, as well as a professor at the Baylor College of Medicine in Houston, Texas—calls the new study “critically important,” stating that it brings together data on the topic, on a global basis, for the first time.

The risk of COVID can be underappreciated by pregnant women, he tells Fortune in a statement.

“Pregnant individuals are typically young and healthy,” he says. “There are, perhaps, misperceptions that they will not ‘get ill’ from this condition. This feeling of being ‘protected,’ coupled with safety concerns (although not founded) of a ‘new’ vaccine, leads” many pregnant women to forgo COVID vaccination.

Virus known to affect the placenta 

The study doesn’t explore the reasons behind the increased risks, Dr. Emily Smith, assistant professor of global health at the George Washington University Milken Institute School of Public Health and the study’s lead author, tells Fortune. 

She speculates that multiple factors are at play, including the virus itself, and immunological and physiological changes that occur during pregnancy. Like the flu, COVID in pregnancy can create issues like pneumonia and the need for ventilation. But it also seems to increase the likelihood of common pregnancy complications like hypertensive disorder and pre-eclampsia, she notes.

Dr. Sarah Mulkey, prenatal-neonatologist neurologist at Children’s National Hospital in Washington, D.C., treats pregnant women with neurological concerns for their fetuses, as well as their babies in the NICU. Some of her pregnant patients have had COVID, and she monitors their children for developmental delays after birth.

Mulkey also considers the elevated risks for pregnant women with COVID multifactorial, and says they extend beyond the virus specifically.

“The maternal immune system is very different during pregnancy,” she tells Fortune. “When a woman gets sick with an infection, she has less of her own immunity to help combat the infection, and can get more severely sick compared to someone who is not pregnant.”

What’s more, COVID is known to affect the placenta, sometimes leading to stillbirth, she adds. Smith’s study found that pregnant women who had experienced COVID had only a slightly elevated risk of stillbirth.

Anecdotally, pregnant women with COVID are often under-treated, Smith says, with medical providers hesitant to recommend new drugs and vaccines, fearing side effects.

The study highlights the need for increased COVID vaccination among pregnant women, and notes that more than 80 countries fail to recommend it.

“There’s a potential risk, of course, with new drugs and treatment,” she says. “But there’s also a risk with disease. I think sometimes people get the risk/benefit calculation wrong, or the data hasn’t been there for it. But this study gives us that data.”