Brain stem abnormalities were identified in babies dying from Sudden infant death syndrome (SIDS).
The researchers analysed the brain stems of 71 infants who had died suddenly and unexpectedly over 11 years.
The study found that all the babies who died had abnormalities of four neurochemicals in the brain stem, located at the skull's base and connects the brain to the spinal cord.
According to Boston Children's Hospital and Florey Institute of Neuroscience and Mental Health's Dr Jhodie Duncan, the research suggests that the abnormality leaves the toddlers unable to adequately respond when faced with a stressor while sleeping.
He said that if a pillow goes over a healthy infant's face, their brain usually detects changes in oxygen levels and initiates response, so that the baby can turn its head and continue breathing. However, babies with the abnormality did not "respond properly" in the same situation, which lead to their death, News.com.au reported.
The next possible step of the research team would be to see if a blood test can be developed to be used as an early screening tool to identify infants at risk of sudden and unexpected death in their first year.
The research has been published in the journal Pediatrics.