In a study that could have important implications for the abortion debate, neuroscientists say they've established that fetuses can't distinguish pain from touch until 35-37 weeks gestation – just before they'd normally be born.
While the issue of fetal pain is by no means the only argument against late abortions, it's become a key issue for anti-abortion campaigners.
Indeed, seven US states have banned abortion after 20 weeks' gestation on the specific grounds that the fetus can feel pain after this point. Idaho resident Jennie McCormack last week filed a lawsuit challenging the constitutionality of the ban.
The University College London team looked at the brain activity of 46 babies, of whom 21 were born prematurely. This allowed them to measure activity at different stages of human brain development, from babies at just 28 weeks of development through to those born 'full term' at 37 weeks.
Using electroencephalography (EEG), they measured the babies' electrical brain activity when they were undergoing a routine heel lance.
In the premature babies, the EEG recorded a response of non-specific 'neuronal bursts' – general bursts of electrical activity in the brain. After 35-37 weeks, though, the babies' response changed to localised activity in specific areas of the brain, indicating that they were now perceiving painful stimulation as separate to touch.
"Premature babies who are younger than 35 weeks have similar brain responses when they experience touch or pain," says UCL's Dr Rebeccah Slater.
"After this time there is a gradual change, rather than a sudden shift, when the brain starts to process the two types of stimuli in a distinct manner."
UCL's Lorenzo Fabrizi cautions that the finding doesn't definitively prove that fetuses don't experience pain at all.
"Of course, babies cannot tell us how they feel, so it is impossible to know what babies actually experience," he says.
Some studies have indicated that fetuses' stress hormones increase when they are subjected to painful stimuli. Once again, though, this doesn't necessarily say anything about how they are actually feeling.