For a new parent, losing a child in their sleep is an absolute nightmare. Not knowing why adds to the anguish.

Thousands of parents in the United States experience this feeling each year after losing a child to sudden infant death syndrome, or SIDS.

SIDS is the “sudden unexpected death of an apparently healthy infant under 1 year of age that remains unexplained,” and the mysterious syndrome is the leading cause of infant death in the United States.

Just under four out of every 10,000 infants in the U.S. die without any explanation, according to a May 25 study published in the Journal of Neuropathology and Experimental Neurology.

Among experts, there is debate about what could lead to SIDS, ranging from serotonin levels in the blood to sleeping positions and heart rate, according to the National Institutes of Health.

“Despite the initial success of national public health campaigns promoting safe sleep environments and healthier sleep positions in infants in the 1990s in the United States, rates of cases have remained the same over the last three decades,” researchers said in a May 25 news release.

With behavioral changes not slowing the number of infant deaths, researchers started examining a potential physical cause.

One group of researchers wanted to take a closer look at a possible biological trigger for some infants to pass away unexpectedly, so they collected tissue samples from the brain stems of 70 infants who died between 2004 and 2011.

The researchers examined the tissue and tested it for potential abnormalities.

SIDS deaths are typically silent, with no struggle, and occur when the infant is asleep or during a period of arousal from sleep, according to the study.

When the researchers looked at the tissue samples, they found that a serotonin receptor in the brain was altered in cases of SIDS compared to infants that died from known causes.

Previous research into the receptor has shown it can contribute to arousal and autoresuscitation, a way the body makes sure the brain gets oxygen as you sleep, according to the study.

By adding a biological element, the researchers created a model to explain how and why SIDS can happen.

First, the child must be in a “critical period of cardiorespiratory development,” which typically occurs during the first year of life for an infant. Second, there must be an outside stressor, such as sleeping in a certain position, that starts to deprive the child of oxygen. Then, lastly, with a biological abnormality making it difficult for the body to know when it’s not getting oxygen, the receptor doesn’t wake the infant up to breathe, and they pass away in their sleep.

“The work presented builds upon previous work by our laboratory and others showing abnormalities in the serotonergic system of some SIDS infants,” study author Robin Haynes said in the release. “Currently, we have no means to identify infants with biological abnormalities in the serotonergic system. Thus, adherence to safe-sleep practices remains critical.”

Until a test can be developed to test for a brain abnormality, the Mayo Clinic shares a few tips to help prevent SIDS in newborns: —Place your baby to sleep on their back until they are physically able to roll over on their own.

—Keep the crib and infant sleeping area bare with a firm mattress.

—Don’t overheat an infant by using an excess of blankets. Instead, try a sleep sack or other sleep clothing that can’t cover an infant’s head.

—Keep the infant sleeping area in the same room as the parents, but in a separate crib or bassinet. Adult beds are unsafe for infants and increase the chance of SIDS.

—Offer a pacifier to the infant without a strap or string that could become tangled.