Assessing a baby's pain can be difficult
Premature babies undergoing medical procedures may be in pain even if there is no obvious physical hint of it, brain scans reveal.
University College London researchers found changes in brain oxygen levels - a response to pain - did not match other signs.
The findings suggest commonly used scales to rate pain in babies may produce misleadingly low scores.
The study is published in the Public Library of Science journal.
The ability of premature babies to feel pain, and the level of painkiller needed to prevent this, has been the subject of debate in medicine.
Some babies do not cry out in response even to procedures such as the "heel prick" test, carried out regularly in neonatal units to obtain a blood sample.
Doctors and nurses frequently have to rely not only on facial changes, and disturbed sleep patterns, but also subtle changes in heart rate and blood oxygen saturation to predict whether an infant is in pain.
The UCL research recorded these responses to the heel prick test in 12 babies while also measuring brain changes in an area of the brain called the somatosensory cortex.
These changes in oxygen levels in certain areas of the brain are believed to be a byproduct of nerve activity in reaction to pain.
While, overall, the relationship between the brain responses and the other changes was close, in some of the babies, there was a brain response without any other physical sign.
Dr Rebeccah Slater, who led the research, said: "Although the study is small, it does raise concerns about the tools normally used by doctors to establish whether a baby is feeling pain.
"Infants may appear to be pain free, but may, according to brain activity measurements, still be experiencing pain."
Andy Cole, the chief executive of premature baby charity Bliss, welcomed the study.
He said: "It is vital for parents and staff to work together to better assess and manage the pain babies may experience during any procedure.
"It is also important to further the understanding of the nature of this pain to reduce the stress that parents feel at this critical time."