This year marks the highest number of measles cases since it was eliminated in 2000. The disease, whose symptoms may include a fever, cough and rash, is highly contagious — and it poses risks to certain populations, including pregnant women. On Wednesday, the Centers for Disease Control and Prevention announced that there are now at least 695 cases in 22 states.

Before a measles vaccine became available in the United States in the 1960s, an estimated 3 to 4 million people were infected each year. By 2000, that number had become negligible thanks to the vaccine, which protects against measles, mumps and rubella (MMR). Public health experts say that most of this year’s cases are cropping up in communities with low vaccination rates, and some attribute the resurgence to misinformation that’s turning parents against vaccines.

Many of the cases are concentrated in New York City. The city’s department of health announced Wednesday that its number of measles cases had risen to 290, including two pregnant women.

“We have now identified two expectant mothers who have contracted measles,” New York City health commissioner Oxiris Barbot said in a statement. “These cases are stark reminders of why New Yorkers must get vaccinated against the measles as soon as possible.”

The department issued an alert Wednesday to health care providers, offering guidance on how to help halt the spread of the illness. The department “specifically alerted” obstetricians and gynecologists and recommended that pregnant women be screened for measles immunity. In cases where pregnant women have been exposed, the statement reads, physicians should quickly determine treatment: “In a past outbreak of measles, cases in pregnant women resulted in a baby being born with measles and a miscarriage.”

The CDC considers pregnant women a high-risk population for complications related to measles. On its site, CDC says that measles may cause women to give birth prematurely or have low-birth-weight babies.

Multiple studies have shed light on the unique risks measles poses for women:

When measles occurs during pregnancy, they’ve found, maternal and fetal deaths increase.

Women are at higher risk for adverse outcomes with many infectious diseases, such as influenza, says Denise Jamieson, professor of gynecology and obstetrics at Emory University School of Medicine. Women are more likely to be hospitalized, to develop pneumonia and to die than nonpregnant women, Jamieson says, because women’s immune systems change when they’re pregnant.

And, when they have measles, “not only are pregnant women more likely to have severe disease, but they’re more likely to have adverse pregnancy outcomes, such as pregnancy loss, preterm birth and low birth weight.”

Jamieson co-authored a paper in 2015 urging health care providers to inform pregnant patients of the disease’s risks, advocate for vaccination and respond quickly in the case of infection.

Kristina Adams Waldorf, professor of obstetrics and gynecology at University of Washington Medicine, has also written about measles and pregnant women. She says that, for any woman, preterm birth is “an incredibly big deal” — an infant might be in the neonatal intensive care unit for weeks and may have developmental delays because of it.

She notes that little research has been done on the long-term effects of measles on fetuses. While doctors don’t believe measles increases the risk of birth defects, she says, they don’t yet know if there’s a higher risk for developing cognitive problems later in life.

Women should not receive the MMR vaccine during pregnancy, according to the CDC. For women who aren’t vaccinated and are exposed to the virus, treatment consists of receiving intravenous immunoglobin, a blood product that helps patients avoid severe disease.

Jamieson advises women who are considering pregnancy, or who want to get pregnant, to find out the status of their measles immunity. If you think you probably received the vaccination when you were younger, that’s not enough, she says; all women considering pregnancy should get tested.

Adams Waldorf tells her pregnant patients to avoid being in an area of outbreak if they can.

“I personally don’t want any of my patients to be exposed to measles when they’re pregnant,” she says.

“As an obstetrician, I default to the most conservative position and say, why expose yourself to communities with large outbreaks if you can avoid it? I think that’s really, really basic.”

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