More than 60 reported cases involve pregnant people, though experts say the official count may be lower than the true number because not every case is reported. Cases have now been identified in 44 counties, including two newly affected counties, according to figures reported Thursday, Oct. 22.

The Pennsylvania Department of Health has issued guidance to health care providers on caring for pregnant patients. Dr. Pamela Bailey, an infectious disease specialist at ChristianaCare, said pregnancy can make people more vulnerable to complications, including miscarriage, stillbirth and premature birth. Babies may face risks from both prematurity and measles itself.

Bailey said measles can cause serious inflammation and breathing difficulties. The MMR vaccine, which protects against measles, mumps and rubella, is not given during pregnancy because it uses a live virus. ChristianaCare doctors are urging people who may become pregnant to check their vaccination records and discuss vaccination with a health care provider if their status is uncertain. The vaccine can be given after delivery and to people who are breastfeeding, the report said.

The outbreak is also affecting southeastern Pennsylvania. Chester County has recorded 92 cases, the state’s third-highest county total, behind Lancaster County’s 457 and Mifflin County’s 193. ChristianaCare has had measles cases at its Chester County and Maryland facilities.

A Centers for Disease Control and Prevention infectious disease expert joined about 300 state health workers this week to help contain the outbreak. Bailey welcomed the state’s guidance, saying measles had been rare enough in recent years that many doctors have limited experience treating it.