Jaime Francis is avoiding all medication during her pregnancy, including Covid-19 vaccines.

Her obstetrician advised her not to take some common painkillers including ibuprofen, she said, and she didn’t fill a prescription she was given for nausea. With the Covid-19 vaccine, the 22-year-old delivery driver from Sparta Township, N.J., said she fears potential long-term developmental effects of the vaccine that might not be known yet. Doctors and researchers say the shots are safe, effective and crucial for pregnant women, who...

Jaime Francis is avoiding all medication during her pregnancy, including Covid-19 vaccines.

Her obstetrician advised her not to take some common painkillers including ibuprofen, she said, and she didn’t fill a prescription she was given for nausea. With the Covid-19 vaccine, the 22-year-old delivery driver from Sparta Township, N.J., said she fears potential long-term developmental effects of the vaccine that might not be known yet. Doctors and researchers say the shots are safe, effective and crucial for pregnant women, who face higher risks of severe Covid-19 than the rest of the population.

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“I don’t see how a vaccine could be safe in the long run if Advil can cause birth defects,” said Ms. Francis, who is expecting her first child, a baby boy, in February. Studies have linked an increased risk of some rare birth defects to the use of common painkillers, including ibuprofen, which is sold under trade names including Advil, during pregnancy.

The Centers for Disease Control and Prevention has recommended since August that expecting mothers or women who plan to get pregnant take the vaccine, citing research showing the shots are safe and effective during pregnancy. Many women in that group haven’t done so.

Some 35% of pregnant women ages 18 to 49 have been fully vaccinated against the illness, the CDC reported last week. That is more than 30 percentage points lower than the 68% of Americans 12 years and older who have been fully vaccinated, according to CDC data.

‘I don’t see how a vaccine could be safe in the long run if Advil can cause birth defects,’ says Jaime Francis, who is expecting a baby boy in February.

Photo: Thalia Juarez for The Wall Street Journal

Doctors at the agency have said they are concerned about slow vaccine adoption among pregnant women. Pregnant women are more likely to experience severe illness from Covid-19 because their immune systems are weakened, research shows, and unvaccinated people have a much higher rate of hospitalization for Covid-19 than fully vaccinated people. Covid-19 can also exacerbate complications such as preterm birth, preeclampsia and babies admitted to the neonatal ICU, according to the CDC.

The CDC said in late September that pregnant women with symptomatic Covid-19 have a more than twofold increased risk of needing ICU admission, ventilation and a blood-pumping device called extracorporeal membrane oxygenation, or ECMO, compared with people who weren’t pregnant, and a 70% increased risk of death from the illness.

The agency has reported more than 140,000 cases of Covid-19 among pregnant women through early November, with more than 24,000 hospitalizations and over 200 deaths.

It is difficult to compare the rate of cases that become hospitalizations between pregnant women and the broader population because testing is often spotty, one reason why epidemiologists believe that a large number of Covid-19 cases go unrecorded.

Women who are pregnant, nursing or hoping to soon become pregnant cite a range of reasons for being apprehensive about the shot, they and their doctors say. On social media, antivaccine advocates have spread misinformation about scientifically unfounded potential consequences of vaccination for women and unborn children, from infertility and miscarriage to unfounded theories that some doctors say patients have mentioned such as blood clots entering breast milk and the vaccines causing the body to magnetize.

Recent studies have shown that the effectiveness of Covid-19 vaccines is decreasing, though experts say the shots still work well. WSJ explains what the numbers mean and why they don’t tell the full story. Photo illustration: Jacob Reynolds/WSJ The Wall Street Journal Interactive Edition

Health officials have struggled to convince expectant mothers that Covid-19 vaccines are safe in part because pregnant women weren’t included in late-stage clinical trials of the three vaccines approved for use in the U.S.

The progress of a Pfizer study focused on pregnant women has been delayed by low enrollment.

“Pregnant women being excluded from the initial vaccine clinical trials left us with basically zero evidence of their safety and efficacy,” said Dana Meaney-Delman, an obstetrician-gynecologist who leads the CDC’s maternal- immunization efforts.

Many of the precautions surrounding medication that some pregnant women follow don’t apply to Covid-19 vaccines, she said. “People see pregnancy as a vulnerable state,” Dr. Meaney-Delman said. “The pendulum has swung too far on the spectrum that women shouldn’t do anything,” with even a tiny risk of complicating a pregnancy, she said.

The World Health Organization at first discouraged use of the Pfizer Inc. -

BioNTech SE shots for pregnant women before later recommending them. Public-health officials in the U.S. at first advised expectant mothers to consult with their doctors to decide whether to get the shots.

An October survey of about 500 women by the virtual healthcare provider Maven found that 61% were unaware of the CDC’s August recommendation to get vaccinated against Covid-19 during pregnancy, and 29% said that a healthcare provider had advised them against getting the vaccine while pregnant.

Some 35% of pregnant women ages 18 to 49 have been fully vaccinated against Covid-19, according to federal data.

Photo: Hannah Beier/Reuters

Non-Hispanic Black women are the least likely to be vaccinated among pregnant people, CDC data show, followed by Latina women. White women and women of Asian descent are most likely to be fully vaccinated.

Ms. Francis, whose family is from Puerto Rico, said she has lost trust in the CDC because of mixed messaging. She also said that she has chosen not to discuss the Covid-19 vaccines with her obstetrician.

“I feel like every week they come out with something different, whether masks work or not, whether something else works,” she said. “Everything’s too up in the air for me to feel confident putting something like that in my body.”

The CDC said public officials need to do more to increase vaccine coverage for pregnant women. “It is challenging to stay on top of all the changing guidance and recommendations as the data accumulates,” the agency said.

Veronica Gillispie-Bell, an obstetrician-gynecologist in New Orleans and medical director of the Louisiana Perinatal Quality Collaborative, a state initiative to improve maternal health, said she has noted a higher degree of distrust of the vaccine among patients of color.

Dr. Gillispie-Bell, who is Black, said she has had the most success persuading patients to get the shot when she acknowledges their concerns and explains that the risks of contracting Covid-19 are much more serious than those the vaccine presents.

Research has shown healthcare providers are less likely to recommend flu vaccines to Black patients, she said. She fears the same might be happening with the Covid-19 shot. Doctors must do more to overcome historically strong distrust of the healthcare system among Black people, she said, and physicians who belong to minority groups have a special role to play in leveraging the trust they enjoy among patients of color to educate them about Covid-19 vaccines.

“I don’t know that we as providers are doing our jobs and advocating for the vaccine as we should,” she said.

Write to Robbie Whelan at robbie.whelan@wsj.com