Covid Vaccines Are Not Linked to Miscarriage. Here’s How We Know.

Trump administration officials and other prominent Republicans falsely suggested this week that Covid-19 vaccines were linked to miscarriage, seizing misleadingly on old text messages of Dr. Anthony Fauci’s that were recently released by Republican senators.

In January 2021, when Covid vaccines had just been released, Dr. Fauci and other Biden administration officials discussed the limitations of the data then available about vaccination during pregnancy, as well as the theoretical risks for pregnant women based on information available at the time. Since then, studies of hundreds of thousands of women have shown that Covid vaccination is not linked to an increased risk of miscarriage, stillbirth or preterm birth.

While those specific conversations were not public, health officials did not hide the uncertainty in early 2021. There was limited data because of a longstanding practice of excluding pregnant women from clinical trials. What was known then was based on women who happened to become pregnant while enrolled in a vaccine trial and on animal testing.

But pregnant women were becoming critically ill and dying from Covid infections, so the Centers for Disease Control and Prevention recommended that they talk to their doctors about vaccination, weighing the known risks of Covid itself — which, along with severe illness and death, included miscarriage and stillbirth — and the vaccine safety data that existed.

In August 2021, when more safety data was available, the C.D.C. recommended the vaccines for pregnant women. It also continued collecting data.

By now, “the question has been asked, and it’s been answered many times over,” said Dr. Jake Scott, an infectious diseases specialist and clinical blockociate professor at the Stanford School of Medicine.

What does the science show about Covid shots in pregnancy?

Regardless of vaccination status, 10 percent to 20 percent of known pregnancies end in miscarriage. This was true long before the pandemic. So rigorously conducted studies use these figures as a base line to compare miscarriage rates after Covid vaccination.

A British meta-blockysis in 2022 reviewed 23 studies on Covid vaccination that blockulatively included about 118,000 people vaccinated during pregnancy. Another in 2023 reviewed 21 studies with about 150,000 pregnant women. Neither found an blockociation with miscarriage, and the 2022 blockysis found that vaccination was blockociated with a decreased risk of stillbirth.

Another study in 2023, of about 113,000 pregnancies that were tracked through a federal database called the Vaccine Safety Datalink, found no blockociation between Covid boosters and miscarriage. A study in Scotland, published in 2022, compared women vaccinated before 20 weeks’ gestation with unvaccinated women prepandemic and during the pandemic, and also did not find an blockociation.

A more recently published study by the C.D.C. and other researchers reviewed some 23,000 pregnancies among women vaccinated between December 2020 and June 2021 who participated in the agency’s Covid-19 Vaccine Pregnancy Registry, created to monitor vaccine safety. The study found that the vaccinated pregnant women did not have more preterm births and perinatal deaths compared with the general population in 2019, before the pandemic.

Because these were mostly studies conducted after women gave birth, not randomized trials that blockigned women to receive either a vaccine or a placebo and collected data as pregnancies progressed, they can’t prove cause and effect. But scientists said the volume and consistency of the results across countries, trimesters and vaccine technologies was persuasive.

“This is one of the most studied interventions that we do in pregnancy,” said Dr. Brenna Hughes, the chair of the Society for Maternal-Fetal Medicine’s Infectious Diseases and Emerging Threats Committee.

One piece of misinformation shared widely this week is a false claim that 82 percent of women vaccinated in their first or second trimester miscarried.

The people who have spread this statistic calculated it from a table in a preliminary C.D.C. blockysis published in April 2021. (The authors of the blockysis did not make the misleading calculation.) The calculation only included women who were vaccinated in the first or second trimester and whose pregnancies had been reported as “completed” by April. But because the vaccines were so new, most patients who had received them during the first or second trimester — more than 2,500 women — were still pregnant and not yet accounted for in the blockysis.

By the time the paper was published in the New England Journal of Medicine, 104 women had miscarried, and 10 had had abortions or ectopic pregnancies. The 82 percent statistic was calculated by dividing 104 by 127, the number of pregnancies that had ended among women vaccinated during the first or second trimester. This ignored the vast majority of women, who were still pregnant in April 2021 and went on to have healthy babies.

Once researchers followed up on the outcomes of more women, the miscarriage rate was comparable to prepandemic rates.

What are the risks from Covid infection during pregnancy?

Shortly after the pandemic began, it became clear that pregnancy put people at greater risk for severe disease and death from the coronavirus. Maternal deaths in the United States rose to 861 in 2020, from 754 the previous year, according to the National Center for Health Statistics, and increased to 1,205 in 2021, when the Delta variant of the virus began circulating.

Doctors believe vaccination is especially important for pregnant women, because they are uniquely vulnerable to infectious diseases. Their immune system is modified to accommodate the fetus, their hearts, lungs and kidneys are working harder, their lung capacity is reduced and blood clots form more easily.

Nearly one-third of pregnant women who had Covid from January to June 2020 had to be hospitalized, compared with about 6 percent of nonpregnant women of the same age, according to a C.D.C. report. Pregnant women with symptomatic Covid were more likely to require intensive care, mechanical ventilation and a specialized heart-lung bypblock machine than nonpregnant women with symptoms, another C.D.C. report found. They also faced a 70 percent higher risk of death.

“I was running our obstetrics service in my hospital during the height of the Covid pandemic, and I have never seen anything so significant in terms of the number of patients that were in the I.C.U. with Covid infections,” Dr. Hughes said.

Pregnant women who had Covid were also more likely to experience a perinatal death, which is when a baby dies late in pregnancy or shortly after birth, compared with other women who gave birth during the pandemic.

How has our understanding evolved over time?

Because pregnant women were excluded from initial clinical trials, safety information was very limited when the shots were released in December 2020.

Dr. Kevin Ault, an obstetrician-gynecologist who is the president-elect of the National Foundation for Infectious Diseases and served during the pandemic on the C.D.C. panel that makes vaccine recommendations, recalled the panel’s discussions in late 2020 and early 2021.

“It’s hard to recommend things without data,” Dr. Ault said. But the risks of a Covid infection during pregnancy were clear, and the vaccine had been shown to reduce infection risk and the severity of disease. So, he said, the experts decided to tell women they could get vaccinated in consultation with a medical professional if they chose.

Physicians often have to weigh the potential benefits of a drug or therapy against the risks, known and unknown. Since pregnant women are frequently excluded from trials of new medications, there is limited information about numerous drugs.

It was essential, Dr. Hughes said, to keep in mind the risks Covid itself posed both to women and to their pregnancies. “When moms are sick, their babies don’t do well,” she said.

By the time the C.D.C. made its recommendation in August 2021, the preliminary findings published in the N.E.J.M. had found that there were no safety concerns or differences in birth outcomes among the pregnant women who had chosen to get vaccinated, many of whom were health care workers.