October, after the last of the tobacco has been harvested and the apple jam canned, Amish communities begin celebrating the months-long “wedding season.”Twice a week, hundreds of celebrants pack into a home to watch a young couple wed and share a meal. These gatherings, however joyful, add to the worries of Roberta Devers, a home-birth midwife in Lancaster County, Pennsylvania. At her prenatal appointments, she’s been asking mothers-to-be: With all of the celebrations, how are you going to keep yourself safe from measles?

In Pennsylvania, at least 1,398 people have been infected with measles this year, according to recent estimates from the state health department; the Amish community, which has significantly lower vaccination rates than other Pennsylvanians, has been especially hard hit. Experts caution that the actual number of infections is likely much higher. “Every house that I go into, either my pregnant mama has it, her husband has it and she’s going to get it, or a family member has it,” Devers told me last week as she drove from one patient appointment to another through Lancaster County’s rolling farmland.

Measles can be especially dangerous during pregnancy, leading to low birth weight, early labor, and miscarriage; one of Devers’s patients recently miscarried at 17 weeks while she was fighting a measles infection.

If more pregnant women get measles, the number of newborns who have measles will almost certainly rise too. According to the American Academy of Pediatrics, historical reports suggest that about a quarter of children born to mothers who are sick with measles in the days leading up to labor will likely acquire the disease. No data on the current rate of measles acquired in utero, which is known as congenital measles, are publicly available yet. But health-care providers working in Pennsylvania say it’s on the rise—perhaps at a scale not seen in decades, if ever.

Before the measles vaccine was licensed in 1963, roughly 95 percent of Americans contracted the disease in childhood, making them immune by the time they entered their reproductive years. But people who are of childbearing age today have lived through historically low measles rates. That means that a number of unvaccinated adults, including pregnant ones, never caught measles in childhood and are therefore immune—and are now contracting the virus during large outbreaks. About two-thirds of the cases reported in Pennsylvania this year have been in adults ages 18 to 49, some of whom appear to be passing on the disease during childbirth.

In an October 19 advisory, the Pennsylvania Department of Health noted that congenital measles had been identified in the state for the first time in decades. “It’s gone from something we never see to something that we are now commonly seeing,” John Goldman, an infectious-disease physician at the University of Pittsburgh Medical Center in Harrisburg, told me. ’s Tom Bartlett reported in August that an infant in Lancaster County, born to a mother who was severely ill with measles, was declared dead within 30 minutes after birth.

Documenting an increase in congenital-measles cases is challenging. Case reports date back to at least the 1930s, but “no one has done any kind of systematic collection” of data, Adam J. Ratner, an infectious-disease pediatrician in New York, told me. The Pennsylvania Department of Health cannot release measles patients’ personal information, including their precise age, in order to protect their privacy, the deputy press secretary Stephanie Nojiri told me, and even if they could, no national definition for congenital measles exists to standardize tracking.

Historically, doctors have diagnosed congenital measles when they observe a measles rash and other typical symptoms of the disease within the first 10 days of a newborn’s life. But that definition may unwittingly count some infants who contract measles in their first days of life outside the womb miss counting infants with milder cases who do not present with a rash, Ratner said.

Because congenital measles has historically been so rare, doctors know very little about its effects. Measles is known to wipe a person’s immune memory, making survivors highly susceptible to infections for years to follow; a 2019 study in children found that the virus demolished anywhere from 11 to 73 percent of antibodies and memory cells. And all people who contract measles are at risk of developing a rare but almost always fatal brain inflammation, subacute sclerosing panencephalitis, that can occur a decade or more after they contract the disease.

But newborns can react to viruses differently than children even a few months older, which could mean that the effects of congenital measles deviate. That dangerous brain inflammation, for example, appears to occur more commonly and more quickly in congenital cases than in cases acquired after birth.

For now, doctors are trying to pool what they do know. Ratner, who is a member of the American Academy of Pediatrics’ committee on infectious diseases, helped assemble new AAP guidance on how to treat congenital measles; he and his colleagues consulted doctors in Canada, which saw congenital-measles cases during an outbreak last year, but also had to extrapolate from how pediatricians treat measles in older children. John Goldman told me that he and other physicians had been following that guidance, as well as the Pennsylvania Department of Health’s, and recommending both vitamin A, which can help limit the severity of an infection in children with a deficiency, and a dose of antibodies, which can help prevent an infected mother from transmitting the virus to her child at all.

“There’s not a lot of good data on how you treat it,” Goldman said. But, he added, Pennsylvania may soon end up with the country’s largest data set of congenital-measles cases, which could help doctors treat those cases more effectively in future outbreaks.

The best way to prevent a congenital-measles infection is, of course, to prevent a mother from getting measles before she gives birth. The best way to do is with the measles vaccine, which is typically given shortly after a person’s first birthday. One of the few times an adult can’t get vaccinated is, crucially, during pregnancy—just when protection is most necessary.

To help shield unvaccinated pregnant women, Roberta Devers argued that the providers who interact with them most often—in many cases, midwives—should be allowed to inoculate the other members of their household. In New York, Governor Kathy Hochul has allowed licensed midwives to administer the measles vaccine as part of a recent disaster declaration responding to the state’s growing outbreak. Pennsylvania, however, does not allow vaccines to be provided by traditional midwives, many of whom study through apprenticeships rather than attending nursing school and are not licensed by the state.

Christian Macedonia, a maternal-fetal-medicine physician in Lancaster County, told me that’s for good reason: Providers who have not gone through formal medical training, he said, should not be distributing shots.

But he also acknowledged that midwives are “really at the front line” of the congenital-measles crisis. In insular religious communities, such as the Amish one in Lancaster County, which have seen the largest measles outbreaks over the past couple of years, midwives may be the only medical professionals with whom families regularly interact. “We have intimate relationships with these families,” Devers said. “We want to be able to carry the vaccine, because our families started to request it.”Devers knows that many of her patients still distrust the formal health-care system.

At a recent postpartum appointment, she talked with a mother who questioned the motives of pop-up vaccination clinics—she was suspicious of the shots being offered for free. As she spoke, the mother cradled her feverish 12-week-old baby, who had measles.