Covid vaccinations are not linked to pre-term births, researchers report.

Women who received Covid vaccinations while pregnant were at no greater risk of delivering their babies prematurely or of giving birth to unusually small babies than pregnant women who did not get vaccinated, a new study reports.

The study, one of the first to examine the health of babies born to women vaccinated during pregnancy, was a reassuring signal. Low-birth-weight babies and infants born early are more likely to experience developmental delays and other health problems.

An earlier study had found that women vaccinated during pregnancy did not face a higher risk of miscarriage than the unvaccinated.

The new study looked at some 46,079 singleton pregnancies that resulted in a live birth, including some 10,064 among women who received one or more doses of Covid vaccine between Dec. 15, 2020, and July 22, 2021, during their pregnancies. Most had received the vaccines made by Pfizer-BioNTech or Moderna, and the vast majority were inoculated during their second or third trimester.

Overall, 6.6 percent of the babies were born prematurely, before 37 weeks of pregnancy, and 8.2 percent were born small for their gestational age, weighing less than 5 pounds and 8 ounces. Researchers found no difference in the rates among mothers who had been vaccinated while pregnant and those who had not.

“We plan to do follow-up studies on infants and their development, but there hasn’t been enough time to do them yet,” said Dr. Heather S. Lipkind, a maternal fetal medicine specialist at Yale University and lead author on the new research.

The study was conducted by the Centers for Disease Control and Prevention, in collaboration with Dr. Lipkind, HealthPartners Institute, Kaiser Permanente researchers and the Harvard Pilgrim Health Care Institute.

The risks of pre-term birth, which appear to be higher when pregnant women are infected with the coronavirus, are well established, she said: “With each week of gestation, the brain develops more. If you have Covid and have to deliver preterm, that can also affect long-term development.”

Another benefit to vaccination is that women may pass on coronavirus antibodies to their infants, helping to protect them from illness, Dr. Lipkind added.

Pregnant women who become infected face a higher risk of developing severe disease, and federal health officials have implored them to get vaccinated. Nevertheless, vaccination rates remain low among pregnant women — about 31 percent as of late September.

Vaccination rates also vary widely by community. While almost half of all pregnant Asian Americans are vaccinated, only 25 percent of pregnant Hispanic women are, and only 15 percent of pregnant Black women, according to the C.D.C.

“I can tell you, anecdotally, that in my practice women who are not vaccinated are for sure getting way sicker,” Dr. Lipkind said, “and we’re seeing pre-term birth in pregnant women who had Covid and loss of pregnancy. It’s very, very tragic.”

She strongly encourages her patients to get vaccinated, especially now that the highly contagious Omicron variant is circulating. But she often faces fierce resistance, she said.

“I think people are just afraid of the unknown in general with pregnancy,” Dr. Lipkind said. “This new research should make people feel a little better about vaccination.”

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