“It’s definitely concerning, given that it’s going in the opposite direction from what it has been,” said Marie Thoma, a University of Maryland researcher who studies maternal and infant mortality.

Dr. Eric Eichenwald, a Philadelphia-based neonatologist, called the new data “disturbing,” but said experts at this point can only speculate as to why a statistic that generally has been falling for decades rose sharply in 2022.

RSV and flu infections rebounded last fall after two years of pandemic precautions, filling pediatric emergency rooms across the country. “That could potentially account for some of it,” said Eichenwald, who chairs an American Academy of Pediatrics committee that writes guidelines for medical care of newborns.