From pacifiers to white noise machines, there is a bevy of products marketed toward notoriously tired new parents that claim to calm babies and help them sleep better.

Among some of the newer offerings are weighted infant sleepwear such as swaddles and sleep sacks that — like weighted blankets for adults — are touted as a way to reduce anxiety and have a soothing effect on the wearer.

But a growing number of doctors and safe sleep advocates are warning about the potential dangers of weighted sleepwear for infants. Babies' bodies are still developing, critics say, and the added heft of weighted clothing could make it harder for infants to breathe, pump blood and move around.

"We do recommend swaddling. We do recommend pacifiers. We recommend rocking and shushing," Dr. Rachel Moon, a pediatrician at the University of Virginia, told NPR. "There are a lot of things that you can do, but please don't use a weighted blanket or swaddle."

Moon is chair of the American Academy of Pediatrics' task force on Sudden Infant Death Syndrome (SIDS), which last year said weighted swaddles are "not safe" and recommended against using them and other weighted infant sleep products.

Though some U.S. safety standards for children's sleepwear exist, such as flammability requirements, there's no standard dictating whether infant sleepwear can be weighted or how much weight can be used. There's also almost no research on the safety or efficacy of weighted infant sleepwear.

The Consumer Product Safety Commission says it received at least one report of a fatality involving a weighted infant sleep product, but it has not named the product. Other reports to the commission and online customer reviews indicate some babies had trouble moving in weighted sleepwear or had the weighted material ride up to their necks.

Still, companies that sell weighted sleepwear for babies say they use a safe amount of weight in their garments and suggest there is little evidence to back up the idea that their products are hazardous.

"Our products are helping a lot of babies and parents," Manasi Gangan, founder and president of the infant sleepwear company Nested Bean, told NPR. "Those who have used [them] swear by our products."

Weighted infant sleepwear, explained

Swaddling young babies while they sleep creates the feeling of being in the womb and helps them avoid accidentally waking themselves up with an involuntary reflex.

Once they start to roll over, many babies transition to sleep sacks, which keep them warm without the use of an actual blanket. Experts say infants should sleep alone in their cribs, since loose bedding can put them at risk of entrapment, suffocation and strangulation.

In recent years, companies have been offering swaddles and sleep sacks with added weight that they say will help ease babies into a restful sleep.

Gangan started Nested Bean in 2011 after the birth of her second son, who would only sleep when Gangan had her hand on his chest. She said the company's weighted sleep sack was the first one to hit the mainstream market, and to date Nested Bean has sold more than 2.5 million products. The company conducted two safety studies on its products before beginning to sell them, and Gangan said the company uses data to guide all of its decisions about safety.

The company's weighted swaddles and sleep sacks, which have added mass in the center and on the sides, are meant to mimic the pressure babies feel when you put your hand on them or hold them.

Though Nested Bean's products vary in weight, its sleepwear for infants under age 1 has roughly 1 to 2 ounces of weighted beads on the chest, Gangan said. The company has guidelines suggesting which weighted products are appropriate for babies based on their weight, and doesn't recommend any of its weighted sleepwear for babies under 6.5 pounds.

Another leading seller of weighted swaddles and sleep sacks is Dreamland Baby, which says its products have "evenly distributed weighted pockets across the front of the sleep sack from shoulders to toes."

Dreamland's weighted swaddles and sleep sacks for infants up to 1 year old contain between 0.8 and 1.5 pounds of weighted beads, according to the company's website, plus the weight of the fabric. The company recommends that babies using those products weigh at least 8 and 15 pounds, respectively.

In a statement to NPR, Dreamland said it has sold more than half a million sleep sacks since 2019 "with no reported adverse events caused by our gently weighted sleep solutions, evidencing that our product is safe."

The company suggested that the American Academy of Pediatrics' guidance against weighted sleep products was "not supported by scientific evidence, including product testing" and urged the organization to reconsider.

Experts say caregivers should wake up to the risks

Weighted infant sleepwear is a "very recent phenomenon" that poses several concerns for young babies, according to Moon.

One is physiological. Moon said infants' rib cages are not entirely bone yet, but rather connective tissue known as cartilage, which doesn't hold its shape as well as bone. "So if you put a weight on it, it's going to deform, and it's going to press down on the lungs and on the heart."

There is also no universal standard for how much weight an infant can handle, though many weighted blankets for older children and adults say users shouldn't exceed a weight of 10% of their body weight.

Moon also worries that weighted sleepwear could cause infants to sleep too deeply. It's important that babies wake up every two or three hours to eat for the first few months of life, she said, and if a baby is sleeping too deeply they may be unable to startle and rouse themselves in a potentially deadly situation, such as not getting enough air.

"Some parents like it because the baby will sleep longer and so the parents can sleep longer, but the sleeping longer is actually what is dangerous," she said.

Experts say weighted swaddles and sleep sacks may also make it harder for infants to move if they need to reposition themselves, such as rolling over from their stomach to their back.

Michelle Barry, the founder and president of the nonprofit Safe Infant Sleep, says she understands why exhausted new parents look for ways to help their babies sleep longer, but she worries about unregulated products that claim to offer a fix.

"We live in a time where, for the most part, if you want it you can find it. The products that are out there are overwhelming," she said. "It's great that we have so many options, but at the same time it's also worrisome."

There's some scientific research, but critics and advocates see the results differently

There has been some research on the effectiveness of weighted blankets in reducing anxiety and insomnia in adults, but the results are inconclusive. Some people with autism and attention deficit hyperactivity disorder have been treated with weighted blankets, too, but Moon says the data there is minimal.

Similarly, there has been scant research done to indicate whether weighted sleepwear is effective for infants — let alone safe.

One study published in 2020 assessed the use of weighted blankets on 16 infants being treated in a neonatal intensive care unit, or NICU, for neonatal abstinence syndrome. Also called neonatal opioid withdrawal syndrome, the affliction affects babies who are forced to withdraw from drugs they're exposed to in the womb, such as opioids or antidepressants, and can cause increased fussiness and trouble sleeping.

Infants in the study were swaddled and then had a 1-pound weighted blanket placed on top of them for 30-minute sessions. Researchers remained in the room during the sessions and monitored the babies' heart and respiratory rates. The study included data from 67 total sessions.

There were no "adverse events" during the study and researchers never had to remove the weighted blankets due to infant distress, they said. The babies experienced "[n]o significant difference" in their breathing rates, but their mean heart rate "decreased significantly" when placed under a weighted blanket versus a non-weighted blanket, the authors wrote.

"This pilot study provides preliminary data that weighted blankets can be used easily and safely in a controlled environment with infants with [neonatal abstinence syndrome]," they added.

In separate research commissioned by Nested Bean and carried out by the engineering firm Engenium, five infants under 6 months of age had their breathing rate, heart rate and oxygen saturation level measured while researchers applied different weighted loads to their chests — roughly 1 ounce, 3 ounces and 9.5 ounces — for two minutes each.

As the weight increased, their oxygen saturation levels dropped, the study found, though it also noted that the average oxygen levels remained within normal parameters. Additionally, the babies' heart rates went up, unlike in the NICU study.

Researchers for Nested Bean said the 1-ounce weight did "not present clear indications for or against noted breathing hazard potential" on the infants in the study, but added that the 3- and 9.5-ounce weights "may increase potential hazard and subsequent risk" due to decreased oxygen levels and heightened pulse rates.

Moon acknowledged the 2020 NICU study but said American Academy of Pediatrics officials could not find any other studies with a "generalizable" population of babies who did not have that illness, and she said the short sessions used in the studies — two minutes and 30 minutes — didn't explain how babies would respond to the products over a longer period of time.

Gangan said she intends to use the results of the company's research to conduct an overnight sleep study.

Regulation hasn't caught up to weighted infant sleepwear — yet

Though there is no standard for weighted infant sleepwear in the U.S., there's work being done to create one.

A group of advocates, industry leaders and others has formed a subcommittee at ASTM, the international voluntary standards body, to create a voluntary standard for wearable infant sleep products, including weighted sleepwear.

The subcommittee is chaired by Barry and Dreamland Baby's CEO, Tara Williams.

Consumer Product Safety Commission officials are also involved in the work of the ASTM subcommittee. Though the CPSC sometimes creates its own mandatory standards for consumer products, it also regularly adopts voluntary standards created by organizations such as ASTM.

In June, American Academy of Pediatrics President Sandy Chung sent the commission and ASTM a letter urging them not to develop a voluntary standard for weighted infant sleepwear. The letter cited a lack of peer-reviewed data and other, non-peer-reviewed data showing a decrease in oxygen levels in infants using weighted sleepwear.

"This means there is evidence that the use of weight sleep products on infants can lead to lower oxygen levels, which if sustained, may be harmful to the developing infant's brain," Chung wrote. "A lack of substantial evidence about the possible harms of weighted sleep products should not serve as evidence that they do not cause harm."

The letter was first reported by NBC.

A spokesperson for the CPSC said it was aware of the American Academy of Pediatrics' recommendations from last year and that it appreciated "the safe sleep guidance these doctors are providing."

The commission also "continues to urge parents to shop carefully and consult with their pediatrician before buying any product that claims to improve baby health or help with sleep," the statement added.

Gangan, for her part, said she supported the creation of a standard for companies looking to bring their weighted infant sleepwear to market.

"The barrier of entry today is extremely low, and that's part of the problem," she said. "So on one hand, someone may take an extremely studious approach. On the other hand, someone may not because they're guided more by intuition than data."

Barry agreed, but added that the opinions of medical experts had to be taken seriously in the creation of a new standard to avoid allowing hazardous products to be sold to unwitting consumers.

She recalled the case of the now-recalled Rock 'n Play sleeper, which a congressional investigation found remained on the market even after Fisher-Price received multiple warnings about its safety.

"I really worry that if we don't fix the process and if we don't listen to these professionals — who have no stake in it in any other way besides not wanting anything to happen to these babies — that we're just going to end up with another Rock 'n Play situation," Barry said.