The nonexhaustive list of things women are told to avoid while pregnant includes cat litter, alfalfa sprouts, deli meat, runny egg yolks, pet hamsters, sushi, herbal teas, gardening, brie cheeses, aspirin, meat with even a hint of pink, hot tubs. The chance that any of these will harm the baby is small, but why risk it?

Yet few doctors in the U.S. tell pregnant women about the risk of catching a ubiquitous virus called cytomegalovirus, or CMV. The name might be obscure, but CMV is the leading infectious cause of birth defects in America—far ahead of toxoplasmosis from cat litter or microbes from hamsters. Bafflingly, the majority of babies infected in the womb are unaffected, but an estimated 400 born with CMV die every year. Thousands more end up with hearing and vision loss, epilepsy, developmental delays, or microcephaly, in which the head and brain are unusually small. Exactly why the virus so dramatically affects some babies but not others is unknown. There is no cure and no vaccine.

Amanda Devereaux’s younger child, Pippa, was born with CMV, which caused damage to her brain. Pippa is prone to seizures. She could not walk until she was 2 and a half, and she is nonverbal at age 7. “I was just flabbergasted that no one told me about CMV,” says Devereaux, who is now the program director for the National CMV Foundation, which raises awareness of the virus. The nonprofit was founded by parents of children with congenital CMV. “Every single one of them says, ‘Why didn’t I hear about this?’” Devereaux told me.  

One reason that doctors have hesitated to spread the word is that the most obvious way to avoid this virus is to avoid infected toddlers. Symptoms from CMV are usually mild to nonexistent in healthy adults and children. Toddlers, who frequently pick up CMV at day care, can continue shedding the virus in their bodily fluids for months and even years while totally healthy. “I’ve encountered a classroom of 2-year-olds where every single child was shedding CMV,” Robert Pass, a retired pediatrician and longtime CMV researcher at the University of Alabama, told me when we spoke in 2021. (He recently died, at age 81.)

This creates a common scenario for congenital CMV: A toddler in day care brings CMV home and infects Mom, who is pregnant with a younger sibling. One recent study found that congenital CMV is nearly twice as common in second-born children than in firstborns. Devereaux’s toddler son was in day care when she was pregnant. “I was sharing food with him because he would not finish his breakfast,” she told me. She had no idea that his half-eaten muffin could end up harming her unborn daughter. In hindsight, she says, “I wish I had spent less time worrying about not eating deli meat and more time focused on, Hey I’ve got this toddler at day care. I’m at risk for CMV.”  

CMV is such a tricky virus because few things about it are absolute. A mother cannot avoid her toddler categorically. Most pregnant women infected with CMV do not pass it to their babies. Most infected babies end up just fine. Doctors warn patients against many risks in pregnancy—see the list above—but in this case thousands of parents every year are blindsided by a very common virus. No one has a perfect answer for how to stop it.

Day cares have been known as hot spots for CMV since at least the 1980s, when Pass, in Alabama, and other researchers in Virginia first began tracking congenital cases back to child-care centers. The virus is rampant in day cares for the same reason that other viruses are rampant in day cares: Young children are born with no immunity, and they aren’t very diligent about avoiding one another’s saliva, urine, snot, and tears, all of which harbor CMV. Of mothers with infected toddlers in day care, a third who have never had the virus catch it within a year. And getting CMV for the first time while pregnant is the riskiest scenario; these so-called primary infections are most likely to result in serious complications for the fetus. But recent research has found that reinfections and reactivations of the virus can lead to congenital CMV too. (CMV remains inside the body forever after the first infection, much like chickenpox, which is caused by a related virus.)

So eliminating the risk of congenital CMV entirely is impossible. But some CMV experts advocate giving women a short list of actions to reduce their risk during the nine months of pregnancy: Avoid sharing food or utensils with toddlers in day care; kiss them on the top of the head instead of on the mouth; wash your hands frequently, especially after diaper changes; and clean surfaces that come in contact with saliva or urine. A study in Italy found that pregnant women who were taught these measures cut their risk of catching CMV by sixfold. A study in France found that it lowered risk too.

In the U.S., patients are unlikely to hear this advice from their obstetricians, though. The American College of Obstetricians and Gynecologists doesn’t recommend telling patients about ways to reduce CMV risk. According to ACOG, the evidence that behavioral changes can make a difference—from just a handful of studies—is not strong enough, and the organization sees downsides to the approach. Advice such as not kissing babies and toddlers could harm “a mother’s ability to bond with her children,” and these hygiene recommendations could “falsely reassure patients” about their risk of CMV, Christopher Zahn, ACOG’s interim CEO, said in a statement to The Atlantic.

The CMV community disagrees. “I think they’re being a bit paternalistic,” says Gail Demmler-Harrison, a pediatric-infectious-diseases doctor at Texas Children’s Hospital. A group of international CMV experts, including Demmler-Harrison, endorsed patient education in a set of consensus recommendations in 2017. Devereaux, with the CMV Foundation, frames it as a matter of choice. It shouldn’t be “somebody else is saying, ‘You can't handle this information; I'm not going to share that with you,” she told me. Without knowing about CMV, women can’t decide what kind of risk they’re comfortable with or what kind of hygiene changes are too burdensome. “It’s your choice whether you make them or not,” she says. “Having that choice is important.”

More data on how well these behavioral changes work might be coming soon: Karen Fowler, an epidemiologist at the University of Alabama at Birmingham, is enrolling hundreds of pregnant women in a clinical trial. Only 8 percent of participants had heard of CMV before joining the study, she says. Patients get a short information session about CMV and then 12 weeks of text-message reminders. Importantly, she says, “we’re keeping our message very simple”: Reduce saliva sharing: no eating leftover food, no sharing utensils, and no cleaning a pacifier in your mouth. This simple rule cuts off the most probable routes of transmission. Sure, CMV is also shed in urine, tears, and other bodily fluids—but mothers aren’t routinely putting any of those in their mouth.

Prevention of CMV ends up the focus of so much attention because once a fetus is infected, the treatment options are not particularly good. The best antiviral against CMV is not considered safe to use during pregnancy, and another antiviral, although safer, is not that potent. After infected babies are born, antiviral therapy can help preserve hearing in those with other moderate to severe symptoms from CMV, but it can’t reverse damage in the brain. And it’s unclear how much antivirals help those with only mild symptoms. When does benefit outweigh risk? “There’s a big gray area,” says Laura Gibson, a pediatric-infectious-diseases doctor at the University of Massachusetts Chan Medical School. For these reasons, policies of whether to screen all newborns vary state to state, even hospital to hospital. Knowledge can be power—but with a virus as confusing as CMV, knowledge of an infection doesn’t always point to an obvious best choice.

In an ideal world, all of this could be made obsolete with a CMV vaccine. But such a vaccine has proved elusive despite a lot of interest. In the U.S., the Institute of Medicine deemed a CMV vaccine the highest priority around the turn of the millennium, and about two dozen vaccine candidates have been or are being studied. All of the completed clinical trials, though, have failed. “The immunity may look robust in the first month or year, but then it wanes,” Demmler-Harrison says. And even vaccines that elicit some immune response are not necessarily able to elicit one strong enough to protect against CMV infection entirely.

CMV is such a challenging virus to vaccinate against because it knows our immune system’s tricks. “It’s evolved with humans for millions of years,” Gibson says. “It knows how to get around and live with our immune system.” Our immune system is never able to eliminate the virus, which emerges occasionally from our cells to replicate and try to find another host. And so a vaccine that completely protects against CMV would need to prompt our immune system to do something it cannot naturally do. It would need to be better than our immune system. “As time goes on, I think fewer and fewer people are thinking that might work,” Gibson says. But a vaccine doesn’t have to protect against all infections to be useful. Because first infections are the riskiest for fetuses, being vaccinated could still reduce risk of congenital CMV.

Whom to vaccinate is another complicated question to answer for CMV. We could vaccinate all toddlers, as we do against rubella, which is also most dangerous when passed from mother to fetus. This has the potential advantage of promoting widespread immunity that tamps down circulation of CMV, period. But the virus doesn’t actually harm toddlers much, and immunity could wane by the time they grow up to childbearing age. Or we could vaccinate teenagers, as we do against meningococcal disease, but teens are more likely to miss vaccines and again, immunity could wane too soon. So what about all pregnant women? By the time someone shows up at the doctor pregnant, it’s probably too late to protect during CMV’s highest risk period, in the first trimester. A better understanding of CMV immunity and spread could help scientists decide on the best strategy. Gibson is conducting a study (funded by Moderna, which is testing a CMV-vaccine candidate) on how the virus spreads and what kinds of immune responses are correlated with shedding.

Until a vaccine is developed—should it happen at all—the only way to prevent CMV infection is the very old-tech method of avoiding bodily fluids. It’s imperfect. Its exact effectiveness is hard to quantify. Some people might not find it worthwhile, given the small absolute risk of CMV in any single pregnancy. There are, after all, already so many things to worry about when expecting a baby. Yet another one? Or, you might think of it, what’s one more?