Sudden infant death syndrome (SIDS) is the unexpected, sudden death of an infant under the age of 1 year with no known cause. The highest risk of SIDS death is between 1–4 months of age.

SIDS is the unexplained and unexpected death of a seemingly healthy infant under the age of 1 year old. SIDS generally occurs when an infant is sleeping.

The Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) states that SIDS is the leading cause of death for infants between 1 and 12 months old. They also note that SIDS causes around half of sudden infant deaths in the United States each year.

Although SIDS can occur at any age below 1 year old, it is more common during certain ages than others. This article will go into detail about the risk of SIDS at different ages, as well as risk factors and prevention.

A timeline identifying when babies are at greatest risk of sudden infant death syndrome (SIDS). Share on Pinterest
Design by Diego Sabogal

SIDS can occur until an infant is a year old. After that, unexplained death is called sudden and unexplained death in childhood (SUDC).

SIDS is more likely to occur at certain ages than at others.

The NICHD notes that SIDS is most common when an infant is between 1–4 months old. Additionally, more than 90% of SIDS deaths occur before the age of 6 months old.

The risk of SIDS reduces after an infant is 8 months old. However, parents and caregivers should maintain safe sleep practices until a child is over a year old.

The cause of SIDS is currently unknown. This makes it difficult for researchers to understand why SIDS is more common at certain ages than others.

Research from 2018 mentions that one theory for SIDS is nervous system “abnormalities.” Changes in the neural control of an infant occur at around 2–4 months, a period when SIDS is most likely to occur.

However, this is currently only a theory, and there is little evidence to confirm it as a cause for SIDS.

Researchers consider SIDS to be the result of many factors, rather than just one. When these factors overlap, an infant may have a higher risk of SIDS.

Possible risk factors for SIDS include:

  • being a male infant
  • being born prematurely
  • having a low birth weight
  • having possible unidentified gene mutations
  • having prenatal exposure to drugs, cigarettes, or alcohol

Certain medical conditions may also increase the risk of SIDS. However, it is inconclusive how important a medical condition is in the development of SIDS.

Researchers theorize that the development of SIDS can occur due to medical conditions that affect the following:

  • respiration function, such as respiratory failure
  • cardiovascular function, which relates to the heart and blood vessels
  • gastrointestinal function, which relates to the digestive system
  • nervous system
  • immune system responses
  • endocrine function, which relates to hormones
  • metabolism
  • biochemistry, which relates to chemical processes in the body

Inadequate nutrition and a buildup of toxins have also been put forward as possible risks for SIDS. However, there is no conclusive evidence that any of these factors cause SIDS.

Other risk factors for SIDS include:

Sleep-related factors

SIDS usually occurs when an infant is sleeping. Researchers consider sleeping position to be a risk factor in the development of SIDS. SIDS is more likely to occur when an infant is asleep on their side or stomach.

A person should place a child on their back when putting them to sleep.

The NICHD also notes that bedding can also be a factor in SIDS. Having blankets, toys, crib bumpers, or other soft objects in an infant’s crib can increase their SIDS risk. Loose objects in a crib have the potential to cause suffocation or entrapment.

Lying an infant on a soft surface, such as a couch or mattress, can increase their SIDS risk. This is because the surface compresses under the infant, causing them to become trapped if they roll onto their stomach.

Altered sleeping patterns may also have an effect on SIDS risk. Altered sleep states may affect an infant’s ability to respond to stressors, which may lead to increased vulnerability during sleep.

Sleeping with another person can also be a factor in SIDS development. If a person falls asleep on a bed or sofa beside an infant, there is a possibility they may roll onto them, causing suffocation. This is also a possibility if a person falls asleep while holding or nursing an infant.

Geographical location and climate

The risk of SIDS can be higher in certain parts of the world.

This could be due to colder climates in certain areas. SIDS is more common during colder months than during the summer. This may be due to a person over-bundling their infant in blankets or clothes, leading to overheating.


SIDS rates can vary depending on an infant’s ethnicity.

Research from 2018 found that SIDS rates were lower in infants with an Asian heritage. Alternately, SIDS rates were higher in infants with an African American heritage or a Native American or Alaska Native heritage.

These variances may be due to geographical location, socioeconomic factors, or differences in child care practices.

It is important to note that there is a disparity in the amount of research relating to health conditions in Communities of Color.

There may be other reasons that SIDS appears to occur more often in certain populations, such as not having the same access to healthcare and mistrust of healthcare professionals due to systemic racism.

Parental characteristics

SIDS is more likely if an infant’s birth parent has:

  • maternal anemia
  • inadequate prenatal care
  • inadequate prenatal nutrition
  • multiple births less than 1 year apart
  • used drugs or alcohol during pregnancy

Being a single parent and giving birth to a child at a young age appear to have an association with higher occurrences of SIDS.

There are various ways a person can help reduce the risk of SIDS, such as by:

  • always lying their infant on their back to sleep
  • keeping soft objects, such as toys and blankets, out of an infant’s crib
  • using a firm, flat sleep surface for the infant
  • breastfeeding or chestfeeding
  • sharing a room with the infant until they are 1 year old, keeping them close but on a separate surface
  • having regular prenatal checks
  • using a pacifier during sleep once nursing is established
  • preventing the infant from overheating during sleep
  • following healthcare guidance on vaccines and checkups
  • allowing the infant to spend time on their stomach, or tummy time, when they are awake and being watched

Parents and caregivers should avoid:

  • products such as wedges or positioners in the infant’s crib
  • smoking or drinking during pregnancy
  • car seats, strollers, swings, or carriers as the infant’s regular sleep spot
Learn more

It is important that an infant has a safe sleeping environment. Learn more about infants and safe sleeping:

SIDS has no warning signs, and infants generally seem healthy beforehand.

A person should contact a doctor if they have any worries or concerns about their infant. Healthcare professionals can discuss the best ways for a person to prevent SIDS from occurring.

If a person notices that their infant is unresponsive or having trouble breathing, they should seek immediate medical attention.

SIDS is the unknown and unexpected death of an infant before they are 1 year old. SIDS has no warning symptoms and can occur in seemingly healthy babies.

Although SIDS can occur at any age before 12 months, it is most common when an infant is between 1–4 months old. SIDS is less common after an infant is 8 months old, but a person should still take precautions to reduce the risk.

Researchers believe SIDS may be the result of several overlapping factors. However, there is no definitive cause.

There are many ways a person can reduce an infant’s risk of SIDS. One of the most important preventive measures is to always lay an infant to sleep on their back.

If a person is concerned about their infant, they should speak to a healthcare professional.