About a third of pregnant 18-to-49 year olds are fully vaccinated as of Oct. 9, according to the CDC. By comparison, 68.1% of people 18 and older are vaccinated as of Oct. 14, the CDC said.

“What I tell my patients is: If what you’re concerned about is doing the thing that is most likely to lead to you being healthy and having a pregnancy without complications, it is unequivocally to receive the vaccine,” said

William Grobman,

professor of obstetrics and gynecology at the Ohio State University College of Medicine and president of the Society for Maternal-Fetal Medicine.

In an interview, Ms. Williams cited a lack of longer-term research on the vaccines’ potential effects. She said that if she got vaccinated and then struggled to conceive, miscarried, or had a child with birth defects, she would always wonder whether the shots caused the problems. “I can’t live with that,” she said. “I can’t live with going through a miscarriage and wondering if it’s because I got an injection to keep my job.” The CDC has said no link has been found between the shots and miscarriage or fertility problems.

ESPN in May told staff who travel to stadium events that they had to receive a Covid-19 vaccine by Aug. 1, according to a person familiar with the matter. The network’s parent

Walt Disney Co.

has required salaried and nonunion office employees in the U.S. to be fully vaccinated by Sept. 30 and currently remote nonunion employees by the end of this month, the person familiar with the matter said.

ESPN is reviewing vaccine accommodation requests on a case-by-case basis and is “granting accommodations consistent with our legal obligations,” a spokesman said. “Our focus is on a safe work environment for everyone.”

Employer vaccine mandates have been met with some mixed feelings, and in sports, a number of athletes and leaders have objected to the measures. On Monday, Washington State fired football coach

Nick Rolovich

for failing to comply with their state’s vaccination mandate.

Pregnant and postpartum women are at increased risk of becoming seriously ill or dying from Covid-19 infections, says the CDC. Their babies are also more likely to be born prematurely and need admission to intensive care units, the agency says.

Cases of Covid-19 among pregnant women rose this summer. More expectant mothers died of Covid-19 in August—22 individuals—than in any other month during the pandemic, according to the CDC. The increase in deaths and the sluggish vaccination rate among expectant mothers prompted the agency to issue an emergency alert in September recommending “urgent action” to boost vaccination among women who are pregnant, breast-feeding, trying to get pregnant or who might be pregnant in the future.

Between Jan. 22 and Oct. 11, there were 128,771 cases of Covid-19 in expectant mothers, 22,538 hospitalizations and 180 deaths, CDC data said.

The CDC has recommended Covid-19 vaccination for pregnant women since April. And the main body for obstetrician-gynecologists in the U.S. has recommended the shots for all expectant mothers since July.

In August, the CDC released the results of an analysis that found that women vaccinated during pregnancy aren’t at higher risk of miscarriage. Among the nearly 2,500 pregnant women in the study who received an mRNA Covid-19 vaccine during the first 20 weeks of pregnancy, the miscarriage rate was 13%. The average rate of miscarriage is between 11% and 16%, says the CDC.

Early research is finding that babies whose mothers received the Covid vaccine while pregnant may receive some protection against infection. A study by researchers at NYU Langone Health in New York found that infants whose mothers received mRNA vaccines had high levels of antibodies generated by the vaccine in their umbilical cord blood. (The researchers were able to distinguish between antibodies from vaccination and antibodies from Covid-19 infection.) The study, which involved 36 newborns, was published in September in the American Journal of Obstetrics & Gynecology MFM.

Ms. Williams’s announcement has drawn attention online. She said she doesn’t know what she will do next, and that she would like to continue reporting.

Concerns about the vaccines’ impact on reproductive health have been partly fueled by the experiences of women who reported that their periods were different after being vaccinated for Covid-19. The National Institutes of Health is funding research looking into possible links between Covid-19 vaccines and menstrual changes.

Ashley Roman,

professor of obstetrics and gynecology at NYU Langone Health and co-author of the newborn antibody study, said pregnant women “are especially sensitive to everything they put in their body.” She said the pregnant patients she sees who don’t take the Covid-19 vaccine believe they can protect themselves with other precautions.

Some still end up infected and sick. “What I hear is, ‘I was doing everything right, I was masking, I was social distancing,’” said Dr. Roman. “At the end of the day they were still able to get the virus.”

Write to Andrea Petersen at andrea.petersen@wsj.com