Babies born during the early months of the COVID-19 outbreak scored slightly lower on a screening of their developmental skills at 6 months of age than those born before the pandemic, according to a new study.

The study in JAMA Pediatrics followed 255 babies born in March to December 2020 in New York City, the U.S. epicenter in the pandemic's early days.

The infants were screened for social, communication, and motor skills at 6 months of age using a standard questionnaire about their ability to roll from their back to their stomach, how often they babble, and other milestones.

In most areas, the babies born during the pandemic had lower scores compared with those born earlier. That held true whether they were born to mothers who had been infected with COVID-19 during pregnancy or not, the researchers found.

"Both exposed and unexposed infants born during that period had significantly lower scores on gross motor, fine motor, and personal-social subdomains compared with a historical cohort of infants born before the onset of the COVID-19 pandemic," the researchers wrote.

"These findings suggest that birth during the COVID-19 pandemic, but not maternal SARS-CoV-2 infection, is associated with differences in neurodevelopment at age 6 months."

But the results might not be indicative of long-term lags, they added.

"It does, of course, give pause: Why the heck are these kiddos scoring less well on what I would consider very foundational skills, like motor skills, particularly?" Sean Deoni, an associate professor of pediatrics research at Brown University who was not involved in the study, told NBC News.

He said he would have expected to see an effect on babies' communication because adults' masks hide their mouths when they talk, and infants have had limited social interaction due to lockdowns.

"My initial impression would be things like language would be affected and motor wouldn't be, and we're seeing the opposite," Deoni told the news outlet.

The researchers speculated stress pregnant women experienced due to the pandemic could explain the drop in babies' motor and social skills.

But lead study investigator Dr. Dani Dumitriu, an assistant professor of pediatrics and psychiatry at Columbia University and a newborn hospitalist at the Morgan Stanley Children's Hospital, told NBC News it was likely due to several factors.

The effects of being born into an environment of lockdowns and other pandemic-related stressors on families probably contributed as well, she told the news outlet — calling the findings a "huge surprise."

"We expected there to be a difference based on what's known from other viruses," she told NBC News.

Mollie Wood, an assistant professor of epidemiology at the University of North Carolina Gillings School of Global Public Health and the lead author of an editorial published alongside the JAMA Pediatrics study, told NBC News pregnant women should not see these findings as reason to opt out of getting vaccinated and the booster shot or to stop wearing masks. She also said the study had limitations.

"It's a very small sample, taken from a very specific health system in New York City, taken during a very limited time period," she told NBC News. "Not that we can't learn from things that way, but it does speak to a very small slice of the pandemic among pregnant people."

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