Brain scans of premature babies undergoing a simple medical procedure have revealed though they may appear to be pain free, they are experiencing pain.
In a study conducted by researchers at University College London researchers found changes in brain oxygen levels which is a response to pain, did not match other signs and while some babies cry and grimace when having the heel prick test, others appear oblivious.
Previous research has suggested that babies who experience a lot of pain in the first few months of life can develop extreme sensitivity to pain as they get older.
The researchers suggest that commonly used scales to rate pain in babies may produce misleadingly low scores.
Lead researcher Dr. Rebeccah Slater says the ability of premature babies to feel pain has been debatable in medicine as well as the level of painkiller needed to prevent pain.
Dr. Slater says some babies do not cry out in response to procedures such as the "heel prick" test, carried out regularly in neonatal units to obtain a blood sample.
Every newborn baby is given the heel prick within the first couple of days of birth and the blood samples are used to detect a range of rare life-threatening diseases including cystic fibrosis, sickle cell disease and congenital hypothyroidism.
All doctors and nurses have to rely on as an indication of pain are facial changes, and disturbed sleep patterns, but Dr. Slater says there are subtle changes in heart rate and blood oxygen saturation which indicate whether an infant is in pain.