SIDS, or sudden infant death syndrome, is the unexplained death of a seemingly healthy baby under one year old. It’s diagnosed only after a thorough investigation, including an autopsy, a death scene examination, and a review of the infant’s medical history, finds no identifiable cause. In 2022, 1,529 infants in the United States died from SIDS, making it one of the leading causes of death in the first year of life.
How SIDS Differs From Other Infant Deaths
SIDS falls under a broader category called sudden unexpected infant death, or SUID. SUID is an umbrella term covering all sudden, unexpected infant deaths where the cause isn’t immediately obvious. It includes SIDS, accidental suffocation and strangulation in bed, and deaths from undetermined causes. About 3,700 SUID deaths occurred in the U.S. in 2022.
The key distinction: SIDS is a diagnosis of exclusion. It’s only used when every other possible explanation has been ruled out. If investigators find evidence that suffocation caused the death, for example, the death is classified differently. In practice, the line between these categories can be blurry, since the classification often depends on how investigators interpret risk factors found in the sleep environment rather than clear-cut physical evidence from an autopsy.
Why SIDS Happens
There is no single known cause of SIDS, but researchers have developed a widely accepted framework called the Triple Risk Model. It proposes that SIDS occurs when three factors overlap at the same time:
- A vulnerable infant. The baby has an underlying condition, often undetectable, such as a brain abnormality or genetic issue affecting heart rate or breathing. Parents and doctors have no way to screen for these vulnerabilities.
- A critical developmental period. During the first six months of life, babies experience rapid changes in blood pressure, breathing patterns, and sleep regulation. Their bodies can be unstable and react in unexpected ways during these windows.
- An outside stressor. Something in the environment, like sleeping face-down or exposure to secondhand smoke, places additional stress on the baby’s body.
When all three circles overlap, a baby who might otherwise survive a minor breathing challenge during sleep cannot recover.
The Brain Chemistry Behind It
One of the most significant research findings involves a chemical messenger in the brainstem called serotonin. In healthy infants, serotonin helps regulate breathing, heart rate, blood pressure, and the ability to wake up when oxygen drops too low during sleep. This waking response, called autoresuscitation, is essentially a built-in safety mechanism. A baby gasps, turns their head, or cries to restore normal breathing.
In many SIDS cases, this system appears to malfunction. Research published in PLOS ONE found that SIDS infants had abnormal serotonin receptor activity in several key areas of the brainstem. In one region that controls the upper airway during sleep, receptor binding was 51% higher in SIDS infants compared to controls. That might sound like “more is better,” but in this case, the receptors involved are inhibitory. More activity means the airway muscles relax too much, increasing resistance to airflow during sleep.
Similar abnormalities were found in brainstem regions that regulate heart rate and blood pressure. The net effect: when a vulnerable baby encounters a breathing threat during sleep (like ending up face-down on a soft surface), their brain may fail to trigger the alarm signals that would wake them up or correct the problem.
Known Risk Factors
SIDS rates are highest in the first six months of life. Several factors increase the risk, and most of them relate to the sleep environment or prenatal exposures.
Stomach sleeping is the single most well-established risk factor. When the “Back to Sleep” campaign launched in the 1990s encouraging parents to place babies on their backs, SIDS rates dropped dramatically. Prone sleeping is thought to increase the risk of rebreathing exhaled air (which is low in oxygen and high in carbon dioxide) and may also trigger dangerous drops in blood pressure in babies with brainstem vulnerabilities.
Smoking is another major risk factor, both before and after birth. The risk increases with the number of cigarettes smoked during pregnancy, the number of smokers in the household, and how close the smoking happens to the infant. An estimated 14% to 16% of all SIDS deaths could be prevented by eliminating secondhand smoke exposure alone.
Other risk factors include premature birth, low birth weight, bed-sharing (especially on soft surfaces like couches or armchairs), overheating, and soft bedding or objects in the sleep space.
Safe Sleep Guidelines
The American Academy of Pediatrics recommends a specific set of practices to reduce the risk of sleep-related infant death. The core rules are straightforward: place your baby on their back for every sleep, use a crib, bassinet, or portable play yard with a firm, flat mattress and a fitted sheet, and keep the sleep space completely clear of loose blankets, pillows, stuffed animals, and crib bumpers.
Your baby should sleep in their own space, not in bed with you, and never on a couch, armchair, or in a seated device like a swing or car seat (unless the car seat is actually in the car). Room-sharing without bed-sharing is ideal for the first six months.
Weighted blankets and weighted swaddles have been specifically flagged as dangerous. The Consumer Product Safety Commission and the AAP both warn against using these products on or near a sleeping infant. Studies have shown they reduce oxygen levels in infants, which can harm a developing brain if sustained. Major retailers have begun pulling these products from shelves following years of advocacy from pediatric groups.
What Parents Can Control
The unsettling reality of SIDS is that some of the underlying vulnerability, particularly brainstem abnormalities, cannot be detected or prevented with current medicine. There is no test, monitor, or device that can reliably predict or prevent SIDS. Products marketed with that claim have no proven benefit.
What you can control is the sleep environment and prenatal exposures. Avoiding smoking during pregnancy and keeping your home smoke-free after birth, placing your baby on their back on a firm and empty sleep surface, and avoiding overheating (dress your baby in no more than one layer beyond what you’d wear) collectively address the “outside stressor” component of the Triple Risk Model. For babies who do have an underlying vulnerability, removing that third factor may be the difference that keeps them safe through the highest-risk months.