The Pennsylvania Department of Health issued an alert Monday to doctors about an increase in measles infections in pregnant women, as well as a concerning rise in congenital measles — a disease that’s rarely, if ever, seen in the U.S. The department said that 66 pregnant women in Pennsylvania had been diagnosed with measles since the state’s outbreak began in April. “Getting measles while pregnant increases the risk of miscarriage, early labor, and low birth weight,” a department spokesperson said in an email.

It’s unclear how many women have miscarried or delivered babies with congenital measles. The department said that state law prohibits it from releasing details of the outcomes of those pregnancies, and there is no national database that tracks cases. True numbers may also be obscured as the outbreak has largely been limited to Amish families who don’t always seek conventional medical assistance. As of Thursday, there have been 4,410 measles cases in the U.S. this year. In Pennsylvania, there have been 1,286 cases as of Monday.

Even before widespread measles vaccination, congenital measles was rare. That’s because most women had already been infected as children. It’s only now emerging as a threat as measles outbreaks are increasing as vaccination rates fall. Last month, the American Academy of Pediatrics added a “frequently asked questions” page about congenital measles to its website in response to ongoing and increasing measles outbreaks. According to the AAP, congenital measles occurs in newborns whose mothers have been infected during the two weeks before birth and who test positive or develop measles symptoms within 10 days of delivery.

Anecdotally, doctors have said that at least six babies were born in Pennsylvania with congenital measles. Babies can have a range of symptoms, from a rash and fever to pneumonia, neurological problems and low blood platelets. At least two developed pneumonia and were admitted to an intensive care unit, doctors said.

Congenital measles can be deadly. One father in Lancaster County previously told NBC News that his wife was sick with measles in August and went into labor a month early. Their baby died shortly after birth. Congenital measles can also lead to severe brain and immune system issues later in life, the Pennsylvania Department of Health said. Those long-term complications can include a very rare and severe inflammatory brain condition called subacute sclerosing panencephalitis, or SSPE.

It can occur seven to 10 years after a measles infection and is almost always fatal. There is evidence that SSPE may occur earlier in children born with congenital measles. There’s no specific treatment for congenital measles other than supportive care, such as fluids, hydration and supplemental oxygen. The best way to prevent the newborn condition is for women to get vaccinated before getting pregnant, the health department said. Pregnant women who get measles are unable to get the vaccine because the shot contains a weakened but live version of the virus.

Unvaccinated pregnant women who are exposed to measles can get a dose of immunoglobulin within six days of that exposure, the health department said, to “provide protection against measles infection or reduce the risk of serious illness among susceptible people.”The vaccine can be given to women after they deliver their babies and is safe for breastfeeding mothers, according to state health officials.