Pregnant women who get mRNA vaccines pass high levels of antibodies to their babies, according to a study published in American Journal of Obstetrics & Gynecology - Maternal Fetal Medicine on Wednesday.

The study — one of the first to measure antibody levels in umbilical cord blood to distinguish whether immunity is from infection or vaccines — found that 36 newborns tested at birth all had antibodies to protect against COVID-19 after their mothers were vaccinated with shots from Pfizer-BioNTech or Moderna.

"We didn't anticipate that. We expected to see more variability," said Ashley Roman, an obstetrician at NYU Langone Health System and co-author of the study.

The data could help encourage more people to get vaccinated during their pregnancies. Only 30% of pregnant people ages 18 to 49 are vaccinated, according to Centers for Disease Control and Prevention data from Sept. 11, despite growing evidence of prenatal vaccine safety. Given the study's small sample size, the team is now looking at results from a larger group, as well as how long immunization lasts for infants after birth.

"We pushed this data out relatively early because it's a unique finding and it has important implications for care," Roman said. "Right now we're recommending all pregnant women receive the vaccine for maternal benefit."

Pfizer and BioNTech's own study on how their shots affect pregnant people and their babies has been delayed due to slow enrollment, the Wall Street Journal reported Wednesday, citing researchers.

Pfizer "stopped enrollment in the U.S. because of recommendations encouraging vaccination of pregnant women," the drugmaker said in an emailed statement to Bloomberg. It's looking at sites in countries that don't advise pregnant people to get shots for possible study sites, according to the statement.

The researchers studied cord blood of 36 fully vaccinated people to look for antibodies to spike protein, which appears after vaccination or getting sick from Covid, and to nucleocapsid protein, which is only present after getting Covid. Prior studies focused on antibodies to the spike protein.

Among the 36 samples the researchers looked at, 31 tested negative for antibodies to the nucleocapsid protein. In other words, 31 pregnant people developed immunity from the vaccine. The other five weren't tested for nucleocapsid protein, so the researchers can't conclusively say the immunity was from the vaccine or from natural infection.

The findings show "very encouraging levels of antibody in cord blood," said Linda Eckert, an obstetrics and gynecology professor at the University of Washington who wasn't involved in the study. "This is another reason pregnant women should get vaccinated, as we are seeing more disease in younger infants and this is a proactive choice pregnant individuals can make to protect their infants."