These findings may help to improve clinical care for preterm babies.
"Babies can distinguish painful stimuli as different from general touch from around 35 to 37 weeks gestation, just before an infant would normally be born," study researcher Lorenzo Fabrizi of University College London said in a statement.
The researchers studied 46 babies at University College Hospital's Elizabeth Garrett Anderson Wing in Bloomsbury, London. Because 21 of the babies were born prematurely, scientists were able to monitor the different stages of human brain activity from just 28 weeks of development to those born full term at 37 weeks. (Babies' due dates are based on 40 weeks of pregnancy, but babies born even at the 37-week mark are considered full term.)
To determine whether the babies were able to feel pain, researchers relied on recordings of brain activity using electroencephalography (EEG). The scientists noted the babies' electrical brain activity as they underwent a routine heel lance, which is a standard, essential procedure of pricking the baby's foot to collect blood samples for clinical use. A change from general brain activity to a localized brain reaction suggested the baby was experiencing pain.
"Of course, babies cannot tell us how they feel, so it is impossible to know what babies actually experience," Fabrizi said. "We cannot say that before this change in brain activity they don't feel pain."
The researchers noted that as a baby's brain develops, bursts of neuronal activity shift from more general activity to more specific, something they say allows the baby to develop more adult-like responses that are more specific to particular sensory inputs.
"In very young brains, all stimulations are followed by 'bursts' of activity, but at a critical time in development, babies start to respond with activity specific to the type of stimulation," Fabrizi said.
Among the premature babies, the EEG recorded a nonspecific "neuronal burst" response to the heel lance, which are general bursts of electrical activity in the brain. However, after 35 to 37 weeks of development, the babies' responses changed to localized activity in specific areas of the brain. This suggests they were beginning to perceive painful stimulation as separate from touch.
The researchers noted that their findings may have implications regarding the care and development of premature newborns, as these children can often grow up to be either more or less sensitive to pain than others.
The study was published Sept. 8 in the journal Current Biology.