The honest answer is that no single cause of SIDS has been identified, which is part of what makes it so frightening for parents. What researchers have pieced together over the past two decades points to a collision of factors: a hidden vulnerability in the baby’s body, a critical window of development, and an outside stressor like sleeping face-down. When all three overlap, an infant who might otherwise be fine can stop breathing and never wake up. In 2022, 1,529 babies in the United States died from SIDS.
The Triple-Risk Model
The most widely accepted framework for understanding SIDS comes from the National Institutes of Health and involves three overlapping risks. First, the baby has an underlying medical condition that no one can detect, such as a brain abnormality or a genetic problem affecting heart rhythm or breathing. Second, the baby is in a vulnerable developmental period. During the first six months of life, blood pressure, breathing patterns, and sleep cycles are all changing rapidly, and the body can react in unpredictable ways. SIDS peaks between 1 and 4 months of age, and 90% of cases occur before 6 months.
Third, something in the environment adds stress to the baby’s system. That might be sleeping on the stomach, getting tangled in soft bedding, or being exposed to cigarette smoke. The model explains why millions of babies sleep on soft surfaces without incident: most of them don’t have the hidden vulnerability. It also explains why the vulnerability alone isn’t always fatal. It takes the combination of all three circles to produce the worst outcome.
Brain Abnormalities That Prevent Waking Up
The most compelling biological research points to problems in the lower brainstem, the part of the brain that controls breathing, heart rate, blood pressure, body temperature, and the ability to wake up. In a healthy baby, rolling face-down into a mattress triggers a protective response: carbon dioxide builds up, oxygen drops, and the brain forces the baby to turn its head, gasp, or cry. In some SIDS infants, that alarm system appears to be broken.
Postmortem studies conducted at the NIH found that brainstems from SIDS infants contained more serotonin-producing nerve cells than those of infants who died from other causes, but those cells had fewer receptors for serotonin and insufficient serotonin transporters. Serotonin is the chemical messenger that helps regulate breathing and arousal during sleep. Researchers believe the extra nerve cells represent the brain’s attempt to compensate for a serotonin system that isn’t working properly. The result is an infant who, when faced with low oxygen or high carbon dioxide during sleep, simply doesn’t wake up.
Genetic Factors
Some infants carry gene mutations that affect the heart’s electrical system. A study published in JAMA examined mutations in a gene called SCN5A, which encodes a protein critical for maintaining a normal heartbeat. Roughly 2% of SIDS cases in that population-based study had an identifiable defect in this gene, one that caused abnormal electrical currents in the heart. This suggests that for a small but real subset of SIDS deaths, the underlying vulnerability is a heart rhythm disorder that no standard screening would catch.
Genetic research has also examined the immune system. Some SIDS infants appear to carry combinations of gene variants that make their inflammatory response unstable. When these babies encounter even a mild cold, their immune system may overreact in ways that interfere with brain function and breathing.
How Minor Infections Play a Role
About half of SIDS victims showed signs of a slight infection, like a mild cold, in the days before death. On its own, a runny nose isn’t dangerous. But research published in Frontiers in Immunology found that SIDS infants who had mild infections before death showed dramatically elevated levels of an inflammatory signaling molecule called IL-6 in their spinal fluid. Some had concentrations in the same range as infants who died from meningitis or blood infections, despite only having minor cold symptoms.
The connection appears to run through the brainstem. SIDS cases showed increased IL-6 receptor activity on the same serotonin-producing brain cells involved in breathing regulation. In other words, the immune response to a simple cold may directly interfere with the brainstem’s ability to control breathing, particularly when a baby is also sleeping in a risky position. This helps explain why SIDS sometimes follows a few days of sniffles that no parent would consider alarming.
Stomach Sleeping and Rebreathing
Placing a baby face-down on a soft surface is the single most well-documented environmental trigger. The mechanism is straightforward: a prone infant’s face can settle into bedding, creating a pocket where exhaled carbon dioxide accumulates and oxygen is depleted. The baby essentially rebreathes its own stale air. Research from the American Academy of Pediatrics tested commercial mattresses and found that even firm surfaces allowed carbon dioxide to build up past thresholds associated with lethal rebreathing in animal models.
For a healthy baby with a functioning brainstem arousal system, this buildup would trigger waking and head-turning. For an infant with the serotonin abnormalities described above, the rising carbon dioxide never sounds the alarm. The baby quietly suffocates without ever showing visible distress.
Overheating
Being too warm during sleep raises the risk of SIDS. Excess heat increases metabolic demand and can suppress the arousal response that would normally wake a baby when something is wrong. Heavy blankets, too many clothing layers, a warm room, or covering a baby’s head can all contribute. The danger is compounded in infants who already have impaired brainstem function, because the same system responsible for temperature regulation is the one that’s already compromised.
Tobacco Smoke Exposure
Smoking during pregnancy and exposing an infant to secondhand smoke after birth are both strongly linked to SIDS. A meta-analysis of existing studies found that prenatal smoking more than doubled the risk (an odds ratio of 2.25), and postnatal smoke exposure nearly doubled it as well (odds ratio of 1.97). Nicotine and other chemicals in tobacco smoke affect fetal brain development, particularly in the brainstem regions responsible for breathing and arousal. Babies born to mothers who smoked during pregnancy may arrive with a less functional version of exactly the protective systems they need most.
Reducing the Risk
Because the underlying vulnerabilities in SIDS are invisible, prevention focuses entirely on removing outside stressors. The CDC and AAP recommend placing babies on their backs for every sleep, including naps. The sleep surface should be firm and flat, like a safety-approved crib mattress with only a fitted sheet. No blankets, pillows, bumper pads, or stuffed animals should be in the sleep area.
Room-sharing without bed-sharing is recommended for at least the first six months. Babies should not be overdressed or have their heads covered during sleep. Offering a pacifier at nap and bedtime has been associated with lower risk, though the exact mechanism isn’t fully understood. If you’re breastfeeding, waiting until nursing is well established before introducing a pacifier is a reasonable approach. Avoiding tobacco smoke, both prenatally and in the baby’s environment after birth, eliminates one of the strongest modifiable risk factors.
None of these steps guarantee prevention, because the hidden biological vulnerabilities at the core of SIDS can’t yet be detected or treated. But removing the environmental triggers effectively breaks the triple-risk chain for the vast majority of infants.