Crib death is the common name for sudden infant death syndrome, or SIDS, the unexplained death of a seemingly healthy baby during sleep. Most cases happen between 1 and 4 months of age, and over 90% occur before a baby reaches 6 months. In 2022, SIDS accounted for 1,529 infant deaths in the United States.
Why It’s Called a Diagnosis of Exclusion
SIDS is not a single disease with a clear cause. The term was first proposed in 1969 to describe a distinctive pattern: an infant dies unexpectedly during sleep, and no cause can be found even after a thorough investigation, autopsy, and review of the baby’s medical history. When every other possible explanation has been ruled out, SIDS is the diagnosis that remains. This makes it fundamentally different from most medical conditions, where doctors identify a specific problem and name it. With SIDS, the name describes what couldn’t be explained.
What Happens in the Brain
Researchers have identified a key biological clue. Infants who die of SIDS tend to have abnormalities in the brainstem, the part of the brain that controls breathing, heart rate, and the ability to wake up during sleep. Specifically, these babies show lower levels of serotonin and altered serotonin receptors in the brainstem. Serotonin plays many roles in the body, but in this region of the brain, it helps regulate the automatic responses that keep a sleeping infant alive, like gasping for air when oxygen drops or waking up when breathing is obstructed.
In healthy babies, a face-down position or rebreathing of exhaled air triggers an arousal response. The baby stirs, turns their head, or cries. In babies with these brainstem abnormalities, that protective reflex may not fire. NIH-funded researchers have found significant alterations in multiple types of serotonin receptors across the brainstems of SIDS cases, suggesting this is not a single defect but a broader disruption in the brain’s alarm system during sleep.
The Triple Risk Model
The leading framework for understanding SIDS involves three factors that must overlap for a death to occur. First, the baby has an underlying vulnerability, such as the brainstem abnormalities described above. Second, the baby is in a critical developmental period when the brain systems controlling heart and lung function are still maturing. Third, an external stressor is present, like sleeping face down, overheating, or exposure to cigarette smoke. No single factor is enough on its own. A vulnerable baby who sleeps safely through the critical window can survive it. An external stressor poses little threat to a baby whose brainstem functions normally. SIDS happens at the intersection of all three.
Sleep Environment Risks
The external stressors parents can control mostly involve the sleep environment. Babies placed on their stomachs or sides to sleep face a higher risk than those placed on their backs. Babies who usually sleep on their backs but are put down on their stomachs even once, such as during a nap at a relative’s house, are at very high risk.
Soft sleep surfaces are dangerous. Waterbeds, old adult mattresses, couches, and armchairs create a very high risk environment. Soft or fluffy bedding, bumper pads, stuffed animals, and loose blankets in the crib increase the chance of suffocation, entrapment, and SIDS. A firm, flat mattress with a fitted sheet and nothing else is the safest setup.
Overheating is another significant stressor. In one early study of 34 SIDS victims, 19 were unusually hot or sweating when found, and 24 were excessively clothed or overwrapped. The risk increases when heavy clothing or blankets combine with a warm room, sleeping face down, or fever. Dressing a baby in one layer more than an adult would comfortably wear in the same room is a common guideline.
Prenatal Exposures That Raise Risk
What happens before birth matters significantly. Mothers who continued smoking beyond the first trimester had babies with a five-fold increased risk of SIDS compared to babies who were unexposed or whose mothers quit by the end of the first trimester. Continued drinking beyond the first trimester raised the risk four-fold. When a mother both smoked and drank throughout pregnancy, her baby’s risk of SIDS jumped to 12 times higher than unexposed infants. These are among the strongest modifiable risk factors for SIDS, and they likely contribute to the brainstem vulnerabilities that make some babies less able to respond to breathing threats during sleep.
What Reduces the Risk
Back sleeping is the single most impactful change parents can make. The 1994 “Back to Sleep” campaign in the United States led to a greater than 50% decline in SIDS rates, though the rate has since plateaued and SIDS remains the leading cause of death in infants past the newborn period.
Breastfeeding provides a measurable protective effect. A large meta-analysis of 18 studies found that any breastfeeding reduced SIDS risk by roughly 45%. The exact mechanism is not fully understood, but breastfed infants appear to be more easily aroused from sleep than formula-fed infants, and breastfeeding helps protect against viral infections that may stress a vulnerable baby’s system.
Pacifier use at sleep time also cuts risk substantially. Multiple studies and two major meta-analyses consistently found that using a pacifier during sleep reduced SIDS risk by about 50%. Researchers are not certain why, but theories include that the pacifier helps keep the airway open by moving the jaw slightly forward, and that it may alter how deeply the baby sleeps, making arousal easier. The protection applies even if the pacifier falls out after the baby is asleep.
Room sharing without bed sharing offers another layer of protection. Having the baby sleep on a separate, firm surface in the same room as a caregiver makes it easier to monitor and respond to the infant while avoiding the suffocation risks that come with sharing a sleep surface, especially soft ones like couches or armchairs.
Who Is Most at Risk
SIDS peaks sharply in the first four months of life, with 72% of all SIDS deaths occurring in that window. The risk drops significantly after 6 months, though it can occur anytime in the first year. Boys are affected slightly more often than girls. Premature and low-birth-weight babies face higher risk, as do infants exposed to smoke both before and after birth. The risk is not evenly distributed across racial and ethnic groups, with Black and American Indian/Alaska Native infants experiencing disproportionately higher rates, a disparity linked to systemic differences in healthcare access, housing, and stress rather than biology alone.