Preemies begin to feel pain around a woman's 35th week of pregnancy, about two to four weeks before delivery, according to a new study from University College London.
Using EEG, researchers recorded the babies' brain activity in response to pain, comparing their pain responses from a touch and prick on the heels. The findings were published 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," Lorenzo Fabrizi, lead author of the study, said in a statement.
The babies, who were 28 to 35 weeks in the womb, showed the same bursts of brain activity for the touch and the heel lance, but babies at more than 35 weeks' gestation had a greater burst of activity in response to the lance than the simple touch.
The findings may explain why babies born prematurely have an abnormal sense of pain, the authors noted, and the findings could potentially affect treatment and care of preemies.
"Clinical practice changed about two decades or more ago to take into account the pain response of premature infants, and term infants," said Dr. Eliot Krane, a professor of anesthesia and pediatrics at Stanford University School of Medicine. "Clinical practice continues to evolve as we become more cognizant of the deleterious effect of pain in infants."
Fetal pain, an area that experts say is lacking in research because it is difficult to study, has often been a point of tension in the ever-controversial abortion debate. Over the past six years, six states have enacted fetal pain abortion bans in which it is illegal to perform an abortion after 20 weeks. Many anti-abortion rights activists argue that fetuses can feel pain in the womb after 20 weeks of development.
"The findings ... should help inform the pain perception portion of the abortion debate," said Dr. F. Sessions Cole, director of the division of newborn medicine at Washington University School of Medicine at St. Louis. "Although this study specifically addresses brain wave differences between premature and term infants, not fetuses, after [receiving] painful and tactile stimuli, it suggests that brain maturation required for fetal pain perception occurs in late pregnancy, more than 11 weeks after the legal limit for abortion in the United States.
"Although fetal pain perception is a complex phenomenon which we do not yet fully understand, this study raises the possibility that maternal pain relief during abortion may require administration of medications more than fetal pain relief," Cole said.
Although the vital sign changes indicate that fetuses and infants perceive pain, Cole said that studies that correlate EEG findings with physiologic indicators of pain may be limited in scope.
"This study did not incorporate any vital sign monitoring, cry response, or other methods for assessing pain," said Cole.
Not unlike older children and adults, premature infants usually receive general anesthetic for surgical procedures, epidural anesthetic and analgesia, nerve blocks, IV pain killers or oral pain killers, Krane said.
"Based on this study, clinical practice in neonatal intensive care units will focus on continuing to reduce use of painful procedures in premature infants," said Cole. "Also, treatment of maternal pain during abortion procedures may require more careful attention than treatment of fetal pain."