A new study looking at more than 40,000 pregnant women adds new evidence supporting the safety of COVID-19 vaccine during pregnancy.

Pregnant women who contract COVID-19 have an increased risk of disease severity and death. Despite this, only 31% of pregnant people in the United States had received vaccines as of September 2021.

One barrier to vaccine acceptance is the concern that vaccination might disrupt pregnancy.

The findings reported by the Centers for Disease Control and Prevention show that COVID-19 vaccination during pregnancy was not associated with preterm birth or small-for-gestational-age (SGA) when comparing vaccinated with unvaccinated pregnant people.

The trimester when a person received a vaccine and the number of COVID-19 vaccine doses received were also not associated with increased risk of preterm birth or SGA, the researchers found.

Preterm birth (in which babies are delivered earlier than 37 weeks) and SGA (in which babies are delivered smaller in size than normal for the gestational age) have been associated with a higher risk for infant death and disability.

For the new study, the authors used data from eight health care organizations participating in Vaccine Safety Datalink—a CDC project established to monitor vaccine safety—to investigate the risk for preterm birth or SGA among vaccinated and unvaccinated pregnant women aged 16 to 49 years.

Among those included in the study, 10,064 people, or nearly 22%, received at least one COVID-19 vaccine dose during pregnancy, researchers say. Most (98.3%) received vaccination during their second or third trimester; the rest (1.7%) received it during their first trimester of pregnancy. Almost 96% of those vaccinated received a Pfizer-BioNTech or Moderna mRNA vaccine.

To date, a few studies have described outcomes among live births following COVID-19 vaccination in pregnancy, the researchers say. The new findings add to the evidence that COVID-19 vaccination is safe during pregnancy.

Research into the drivers behind low vaccine acceptance among pregnant people has found that the most common concerns have been a lack of information about COVID-19 vaccine safety in pregnant people and potential harm to the fetus.

The results of this study speak to both, says Heather Lipkind, associate professor of obstetrics, gynecology, and reproductive sciences at the Yale School of Medicine and lead author of the study.

“Getting vaccinated against COVID-19 is important for preventing severe illness in pregnant women,” she says. “With the increasing rates of COVID-19 in our community we are encouraging pregnant people to get vaccinated.”

Pregnant women, in addition to seeing an increased risk of severe disease and death compared with non-pregnant people, are more likely to require admission into the intensive care unit, invasive ventilation, and machine-assisted blood oxygenation.

The CDC and the American College of Obstetricians and Gynecologists recommend the COVID-19 vaccine for women who are pregnant, breastfeeding, trying to get pregnant, or might become pregnant in the future.