Sudden infant death syndrome (SIDS), also known as "cot death", can be unexpected and unexplained, but a new study has found a possible factor linked to it.
The first factor was the infant is at a critical stage of development during their first year of life.
Second, the baby is exposed to a stressor, such as sleeping face down, which can prevent the baby from breathing properly and raising the level of carbon dioxide.
Third, the baby has an underlying abnormality that makes it harder to survive the traumatic event.
The team of researchers found that there is a particular brain receptor which helps babies gasp for air. In the case of those that died of SIDS, the receptors were altered in some infants.
The receptor in question is part of the serotonin system.
This plays an important role in regulating involuntary body functions like heart rate, breathing and blood pressure.
How was the study conducted?
The study looked at cases between 2004 to 2011 and examined brain tissue from 58 infants.
They then compared those samples to the brain tissue of 12 infants who died of other causes, such as pneumonia or heart disease.
The results showed that the babies who had died of SIDS were more likely to have an altered version of the serotonin-related brain receptor than the control cases.
Robin Haynes, the study's lead author and a researcher at Boston Children's Hospital, said infants normally have a protective response that prompts them to gasp for air when they don't get enough oxygen during sleep.
"They arouse and they go through what's called auto-resuscitation, where it kick-starts breathing," she said.
However, with SIDS, the infant's response may not kick in due to the alerted brain receptor. This can therefore hinder the blood flow and the baby's oxygen supply.
Ms Haynes also added that although researchers identified "abnormalities" in the serotonin receptor, the link between this and the cause of death remains "unknown".
"Although we have identified abnormalities in the serotonin 2A/C receptor in SIDS, the relationship between the abnormalities and cause of death remains unknown."
She added: "Much work remains in determining the consequence of abnormalities in this receptor in the context of a larger network of serotonin and non-serotonin receptors that protect vital functions in cardiac and respiratory control when challenged."
Ms Haynes urged those caring for young children to maintain "safe-sleep practices".
What causes SIDS?
According to the NHS, the exact cause of SIDS is unknown, but it has given advice on how parents can prevent it. The methods include:
• Always make sure you have placed your baby on their back to sleep.
• Place the baby in the "feet to foot" position - with their feet touching the end of the cot, pram or Moses basket.
• Keep the baby's head uncovered while sleeping, and their blanket should be tucked no higher than their shoulders.
• For the first six months, make sure you are sleeping in the same room as the baby.
• Make sure there are no pillows or duvets near the baby if you share a bed with them.
• Keep other children and pets out of the bed if you share a bed with the baby.
• Use a mattress that is firm, flat, waterproof and in good condition.
The US Department of Health and Human Services has said that in 2020 there were 1,389 deaths due to SIDS in the United States.
In the UK, around 200 babies die suddenly and unexpectedly every year in the UK, the NHS has said.
But SIDS still remains rare, and the risk of your baby dying from it is low.
"Most deaths happen during the first six months of a baby's life. Infants born prematurely or with a low birth weight are at greater risk. SIDS also tends to be slightly more common in baby boys.
"Parents can reduce the risk of SIDS by not smoking while pregnant or after the baby is born, and always placing the baby on their back when they sleep," the NHS website says.