COVID can travel from a pregnant mom to a fetus, breaching the placenta and causing brain damage, according to a study published Thursday in Pediatrics.

Researchers with the University of Miami documented the cases of two infants born to mothers who had COVID during pregnancy. Both infants tested negative for the virus at birth, but experienced seizures, delays in developmental milestones, and a head size that became disproportionately small with time.

Both infants eventually died.

While such outcomes are rare, those who are pregnant or planning to become pregnant need to be cognizant of the risks COVID poses to fetuses, Dr. Shahnaz Duara, a neonatologist at the University of Miami’s Miller School of Medicine and senior author on the paper, tells Fortune.

“Most young people who are child-producing age treat COVID as if it’s going to be a little viral infection, and they don’t want to bother with getting vaccinated or taking precautions,” she says.

“They’re thinking it’s not going to make them sick. And it doesn’t. But it can have a big impact on the baby.”

Medical professionals were already aware that roughly two or three babies out of every thousand are born COVID-positive, with some reports of stillbirth among them. It’s possible that additional babies whose mothers had COVID while pregnant have been born neurologically impacted. Doctors may not have connected the dots because the babies didn’t test positive for the virus at birth, Duara says, or because the neurological issues have yet to surface.

In the cases Duara and his team studied, the virus was discovered in infants’ tissue–a finding that would generally only occur at autopsy, and only if health professionals were looking for it, she notes.

“There may have been other cases like this that have gone unrecognized,” she says. “Others would have said, ‘This can’t be COVID; the babies didn’t swab positive.’ But maybe [the babies] didn’t get studied to the extent we studied” these babies.

Duara hopes the new research raises awareness among parents and medical professionals alike.

Reassuringly, the mothers’ cases appear unusual. One was incredibly sick with COVID for weeks during mid-2020, and the other appears to have been infected twice during her pregnancy during early 2021, but never had symptoms. Both mothers were around 25 weeks of gestation when exposed to the virus, and neither were vaccinated. But other patterns could lead to similar outcomes. That’s what researchers are trying to figure out now, Duara says.

Advice about COVID-related pregnancy complications

His message to moms-to-be: “Use the same precautions you would for your grandparents, for those on steroids or who are immunocompromised,” she says.

Consider getting the COVID vaccine ahead of getting pregnant, or early in your pregnancy.

If you’re able, breastfeed your child at least until 6 months of age, when they’re eligible for their first COVID vaccine. If a mother has immunity to COVID, she can transfer antibodies to her baby via breast milk.

Other precautions, including masking, can help reduce the risk of becoming infected with COVID while pregnant.

When it comes to babies whose mothers had COVID during pregnancy, she recommends parents tell their pediatrician, and report any delays in developmental milestones, as well as behavioral differences and hearing issues.

“The message really is that we don’t know much about this virus yet–we’ve only been at it for four years,” she says. “But we know that in special populations, [the virus] behaves in special ways.”

“We know about the elderly, what it does to them. Babies are also at risk, though we don’t know yet if that risk is across the board, or if there are specific points in gestation that are at risk, or something in [infants’] own data that makes them more vulnerable,” she adds, noting that genetic testing has not yet been performed on infants.

At this point, researchers have more questions than answers, Duara says. But they know one thing: COVID in pregnancy is a problem.

Notes Duara: “Perhaps it’s only a problem for a handful of people, but it’s a problem.”