Babies, who are born prematurely, may not start to feel pain until around 35 to 37 weeks into their development, researchers reported Thursday.

The British scientists studied the brain activity of 46 infants between the ages of 28 and 45 weeks of gestation as they were either tapped gently or jabbed painfully with a lance on their heels. EEG readings found that the babies’ brains did not start to respond differently to the pain of the heel sticks until between about 35 to 37 weeks, the researchers reported in the journal Current Biology.

“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,” said Lorenzo Fabrizi of University College London, who led the study. A full-term pregnancy is generally considered to be about 40 weeks.

The results could have implications for the care of premature infants, Fabrizi said. The findings suggest, for example, that exposing preemies to painful procedures in the hospital could make them hypersensitive to pain later in life.

“Repeated noxious stimulation of the kind used in this study is a feature of neonatal intensive care,” the researchers wrote. “Our finding that noxious heel lance increases neuronal bursting activity in the brain the earliest age raises the possibility that excess noxious input may disrupt the normal formation of cortical circuits, and that this is a mechanism underlining the long-term neurodevelopmental consequences and altered pain behavior in ex-preterm children.”

Some U.S. states have begun restricting abortion on the basis that fetuses may feel pain — an idea that has been challenged by many medical experts. Fabrizi declined to speculate on how his findings might apply to that issue.

“The study is focussed on postnatal infants, so we cannot make conclusions about what happens inside the womb and to fetuses,” Fabrizi wrote in an email.