More parents are refusing vitamin K shots for their newborns, a study published Monday in the Journal of the American Medical Association found. It’s a trend that experts worry could have deadly consequences.

Babies are born with very low levels of vitamin K, a nutrient the body needs for blood to clot, leaving them at risk for severe bleeding early in life. In the early 1960s, hospitals in the United States began giving newborns shots of the vitamin within the first six hours of birth to prevent bleeding, including bleeding in the gastrointestinal tract or brain.

Dr. Kristan Scott, a neonatologist at Children’s Hospital of Philadelphia and the leader of the study, said he and his co-authors noticed a rise in parents turning down the shot in their own practices, prompting the research.

Still, Scott said, he wasn’t fully expecting the results.

“The increase is not surprising, but the degree to which it did increase did catch me off guard,” he said.

The study analyzed electronic medical record data from Epic Systems’ Cosmos database, which included whether an infant received a vitamin K shot. The study included more than 5 million babies born in 403 hospitals in all 50 states, from 2017 through 2024. The researchers found that about 4% of babies — about 200,000 — born in this time frame didn’t get vitamin K shots. The percentage rose from less than 3% in 2017 to more than 5% in 2024. The trend was highest among non-Hispanic white babies.

The rate really started increasing from 2019 to 2020, and accelerated during and after the Covid pandemic, the study found.

Scott said there haven’t been any large policy changes regarding vitamin K shots from hospitals, nor changes in recommendations from medical organizations. That means the rise in babies not getting vitamin K shots is almost certainly due to parental refusal, he said.

Reports of a trend go back before 2017. A 2016 study in the journal Hospital Pediatrics looked at why parents refuse the shot.

Widespread misinformation on social media and rising vaccine skepticism are likely contributing to an uptick in the number of parents refusing vitamin K shots for their newborns, said Dr. Tiffany McKee-Garrett, an associate professor of pediatrics at Texas Children’s Hospital in Houston who wasn’t involved with the new study.

“Parents are equating vitamin K injections to vaccines,” McKee-Garrett said, adding that a vitamin K shot is not a vaccine, but a supplement derived from a plant.

Dr. Ivan Hand, director of neonatology at NYC Health + Hospitals Kings County in Brooklyn, has also noticed the trend in his practice.

“I think this comes from general mistrust in authority that started probably in the later part of the 2010s,” said Hand, who wasn’t involved in the new research.

In 2022, Hand co-authored an American Academy of Pediatrics policy statement on parents refusing vitamin K shots for newborns.

Newborns who don’t get a vitamin K shot are more than 80 times more likely to have bleeding from vitamin K deficiency than those who get the shot, according to the Centers for Disease Control and Prevention. Bleeding can happen up to six months after birth.

“We are creating a population of newborns who are at risk of bleeding,” Scott said, adding that this could include anything from bruising, to losing blood when the umbilical cord is cut, to gastrointestinal bleeding. “Bleeding into the brain is what we really worry about, essentially a stroke,” he said. “That can ultimately cause death.”

It’s unclear whether the drop in the number of infants getting the shot that doctors are seeing correlates to a rise in the number of cases of bleeding from vitamin K deficiency in infants.

Hand said he has not seen an increase in his practice, but added that he only sees infants just after they are born. Scott said he believes further research will find an increase in bleeding events.

“We know unequivocally that when infants do not receive vitamin K, they are at increased risk of bleeding,” he said, adding that at CHOP, there has not only been an increase in vitamin K refusal, but also an increase in bleeding.

In Houston, McKee-Garrett has heard reports of vitamin K deficiency bleeding, something that was extremely rare for decades after the U.S. began widely giving vitamin K shots.

“It’s a preventable disease, so we shouldn’t be seeing it at all,” Hand said.

In some countries, babies are given oral doses of vitamin K supplements. However, these formulas can be inconsistent and each baby’s digestive system absorbs different amounts of the nutrient, Hand said, adding that breast milk is also not a good enough source of vitamin K to prevent deficiency. Oral vitamin K supplements also need to be given multiple times, whereas the vitamin shots only need to be given once. Studies in Europe have also shown that while oral vitamin K supplementation may be effective in the first week of life, it may be much less effective at preventing late-onset vitamin K deficiency bleeding, which can occur up to 6 months of age.

“That late-onset bleeding, the bleeding that occurs internally, has a 20% mortality rate,” McKee-Garrett said. Some studies have found it to be as high as 50%.

Because vitamin K shots have been widely given in the U.S. for more than 60 years, Hand said parents may not understand the risk of forgoing the prophylactic.

“These treatments have been so effective that people don’t understand the consequences. They have never seen babies with severe bleeding, so they think it doesn’t exist,” he said. “But you don’t see it because we are treating these kids.”

This article was originally published on NBCNews.com