When the mother arrived with her newborn, blood oozing from the umbilical-cord stump, Dr. Jessica Kirk and a nurse were alone in a small pediatric emergency room in Florida.

After two decades in medicine, Dr. Kirk, a pediatric hospitalist now practicing in Fairhope, Ala., usually has an idea of possible diagnoses within moments of meeting a patient. But she hadn’t seen this before. As the nurse took vital signs, Dr. Kirk called her medical director.

“Oh no,” she recalled the director saying upon hearing the child’s name. The parents, the director said, had declined the vitamin K injection newborns routinely receive to help blood clot. Without it, infants are vulnerable to spontaneous bleeding.

This baby was hemorrhaging internally.

Within minutes, he was cold, pulse high, blood pressure low, Dr. Kirk recalled. She gave IV fluids, an emergency dose of vitamin K and a plasma transfusion, all while trying to arrange a transfer to a neonatal intensive care unit. The baby’s abdomen became firm from blood flooding into it. He started to fade out of consciousness.

It was 2021, and the number of parents refusing the vitamin K shot had begun to increase, quietly enough that encountering a baby who hadn’t received it still shocked Dr. Kirk. Now, many doctors say they see that regularly, and bleeding cases are accumulating.

A study of electronic medical records found that 5.2 percent of babies in the U.S. went without the shot in 2024, up from 2.9 percent in 2017, meaning tens of thousands more unprotected babies. And more than 15 doctors told The New York Times they had observed a further increase in the past two years.

The resulting hemorrhages aren’t tracked nationally. But experts estimate that as many as one in 60 untreated infants — 1.7 percent — will suffer a bleed in the first week of life, and another fraction of a percent within six months.

More than a dozen doctors around the country — including emergency physicians, neonatologists and pediatricians — told The Times they had treated brain or abdominal hemorrhages in infants who hadn’t received vitamin K, mostly in the past five years. They did not always know the outcomes, since some patients were transferred to other facilities. But at least 12 of these babies died, and at least 14 others had brain damage.

Some doctors described cases in broad terms to protect patient privacy, and some who provided more details asked that babies’ sex or exact age not be published.

Dr. Meghan Martin, a pediatric emergency medicine physician at Johns Hopkins All Children’s Hospital in St. Petersburg, Fla., said she and her colleagues now treated a hemorrhaging infant every month or two on average.

“It’s not unusual anymore,” Dr. Martin said.

On June 29, Senator Angela Alsobrooks of Maryland and Representative Kim Schrier of Washington, both Democrats, wrote to Dr. Jay Bhattacharya, who is currently leading the Centers for Disease Control and Prevention, urging the C.D.C. to publicly track vitamin K refusal and deficiency bleeding. A spokeswoman for the Department of Health and Human Services did not say whether it would.

National data would enable doctors “to sit down with a family and say, ‘Let me tell you what we’re seeing,’” Ms. Schrier, who was a pediatrician before coming to Congress, said in an interview. “It feels like a more real-world discussion than a theoretical risk.”

Several doctors described vitamin K deficiency bleeding cases as so horrific that they were seared in their minds, though they treat sick children every day.

Dr. Donna Schoonover, a pediatric hospitalist in Washington, said she would never forget one image: an eye popping out of its socket from the blood pooling in an infant’s skull.

A Routine Injection

Infants are naturally deficient in vitamin K because it doesn’t cross the placenta well, and breast milk contains little. Formula can contain more — but newborns’ digestive systems can’t fully absorb it, said Dr. Leela Sarathy, the medical director of newborn nursery services at Mass General Brigham for Children.

The risk of bleeding from vitamin K deficiency is highest in the first week but persists at a lower level for months. One injection within six hours of birth nearly eliminates it. (Unrelated factors can also cause bleeding.)

The American Academy of Pediatrics recommended the shot in 1961, and vitamin K deficiency bleeding became very rare in the United States. “We learned about it, but it was something that happened mostly in other countries,” said Dr. Judy Felgenhauer, the medical director of the pediatric hematology and oncology program at Providence Sacred Heart Children’s Hospital in Spokane, Wash.

The injection is very safe. But in the 2010s, doctors started to notice more Americans turning it down for their children. While the vitamin K shot is not a vaccine, rejection of it rose alongside vaccine rejection, among many of the same parents.

Misinformation online has contributed, said Dr. Stephanie DeLeon, the associate chief medical officer at Oklahoma Children’s Hospital OU Health.

Some Facebook groups and other forums for expectant parents are full of inaccurate claims and parents expressing anguished indecision, unsure what to believe and terrified of harming their children.

Though the C.D.C. still recommends vitamin K, Health Secretary Robert F. Kennedy Jr. and other Trump administration officials have amplified distrust of standard medical guidance in many areas. And Children’s Health Defense — an organization Mr. Kennedy previously led — has suggested vitamin K shots could be dangerous.

Emily Hilliard, a spokeswoman for the health department, blamed the Biden administration’s Covid policies for the increase in vitamin K rejection and said the department was seeking to “rebuild” trust “through honesty, informed consent, sound science and respect for individual choice.”

Some parents tell doctors they are worried about a preservative called benzyl alcohol, though there is no evidence of harm from the tiny amount in the vitamin K shot, and preservative-free shots are available. Some believe the shot is linked to leukemia, based on a small study from the early 1990s that multiple larger studies have contradicted.

Other parents oppose interventions that they consider unnatural. Some want to spare their baby the discomfort of an injection, and think the danger of skipping it is insignificant.

“They’ll say it’s a one-in-a-million bleeding risk,” said Dr. Annemarie Stroustrup, the senior vice president of pediatric services at Northwell Health, a large health care network in New York and Connecticut where several infants have hemorrhaged in the past two years. “It’s decidedly not.”

An emergency vitamin K injection can start the clotting process within hours, and some babies can be kept alive until then with transfusions, surgery to relieve pressure on the brain by removing part of the skull, or life support.

But an estimated 20 percent of vitamin K deficiency bleeds are fatal. And many survivors have brain damage, which can cause paralysis, intellectual disabilities or a long-term need for a feeding tube or ventilator.

Dr. Martin said that she had treated more than a dozen infants with vitamin K deficiency bleeds, and that devastated parents often told her they knew there was a theoretical risk but never thought it would happen to their child.

Unforgettable Devastation

The first signs of bleeding can be subtle, doctors said. An infant might be sleepy or not feed normally.

Then “you snap your fingers” and they’re critically ill, Dr. Martin said.

Some babies have seizures. Some vomit or defecate blood. Some stop breathing. They are rushed to the hospital, where scans show profuse bleeding, and tests can distinguish vitamin K deficiency from other causes.

Dr. Jeremy Jacobs, an assistant professor at Vanderbilt University Medical Center who specializes in blood coagulation and recently co-wrote a paper on vitamin K deficiency bleeding, recalled a child a few months old who arrived with bruising and spots of blood under the skin. Imaging showed a brain hemorrhage.

Clotting improved after an emergency dose of vitamin K, Dr. Jacobs said, but it was too late. The baby died.

Conversations with parents can be shattering, doctors said. They emphasized the importance of compassion for people who thought they were doing the right thing.

Some parents struggle to accept a connection to their refusal of the shot. The other response, Dr. DeLeon said, is “utter devastation and guilt.”

On the day the newborn arrived in her emergency room, Dr. Kirk worked single-mindedly until he could be transferred to a larger hospital. The transfer meant she would never know whether the child survived.

As the transport team took charge, she had a moment to speak with the mother, who had been sitting, her face white, watching the doctor and nurse try to save her baby.

Dr. Kirk meets many parents on the worst days of their lives. She looks them in the eye, she said, and tells them: “I just want you to hear this from me and remember my voice saying this to you: This is not your fault. There’s no way you could have seen this coming. There’s nothing you could have done differently.”

She looked this mother in the eye and told her it wasn’t her fault. She meant it. She knew the woman had believed she was acting in her child’s best interests.

“But what I could not bring myself to say to her was, ‘There’s nothing you could have done,’” Dr. Kirk said. “I just couldn’t say it.”