Fetuses can tell the difference between pain and touch in only the last two weeks before birth, which could help to explain why babies born prematurely often have abnormal pain responses.
Lorenzo Fabrizi from University College London and colleagues used EEG, a non-invasive way of measuring brain activity, on 46 newborn babies as they underwent a routine heel lance – a pinprick to the heel for taking a blood sample.
They also measured how the babies’ brains responded to normal touch – a light tap to the heel. Almost half of the babies were born prematurely – some at just 28 weeks – so the team were able to compare the responses of babies in the final stages of development with those of babies born at full term.
Premature babies up to the age of 35 weeks had bursts of activity across the whole brain in response to both pain and touch, but a change happened around 35 weeks. Between 35 to 37 weeks – just before a fetus would normally be born – the brain seemed to become able to tell the two stimuli apart. The responses to both pain and touch now took place in specific areas on the front, back and sides of the brain, but the signal was much stronger for pain.
Welcome to pain
“This is an important stage in the development of the brain,” says Fabrizi, when changes occur to allow the brain to process sensory stimulation in a more sophisticated way in preparation for life outside the womb.
Fabrizi believes that the general bursts of brain activity experienced by developing fetuses are part of that development – they help connections to form between neurons in the brain. That could be a reason to treat premature babies with even more care than usual: “By evoking [bursts of brain activity] when the baby is born prematurely we may be interfering with the normal wiring in the brain,” Fabrizi says, adding that premature babies can be subjected to up to 10 painful procedures a day in hospital.
This might explain why children born prematurely have been found to have abnormal pain responses, although the long-term effects remain unclear, says Fabrizi.
Journal reference: Current Biology, DOI: 10.1016/j.cub.2011.08.010