For most of the 20th century, scientists denied that infants after birth feel pain.1 Thankfully, everyone now accepts that newborns feel pain, but history has left its mark: scientists are still uniquely willing to ignore signs of potential fetal pain.
Prenatal Stress and Pain
Why do scientists disagree about when the fetus starts to feel pain? The answer lies in the fact that there are two different types of pain: the immediate and unreflective experience of pain, and the emotional response to pain. The pain experience can be measured by looking at the activity of pain receptors, while the emotional response to pain needs to be reported, either using a pain scale, or by noticing a child’s response, such as crying.2 Despite the debate, anesthetizing the fetus inside the mother’s womb is now standard procedure in prenatal surgeries.3
Scientific advancements provide clear evidence that the fetus feels pain by 15 weeks gestation, and possibly even earlier. Furthermore, pain causes fetal stress. By 18 weeks, the fetus responds to a painful procedure with a surge in circulating stress hormones separate from the mother’s hormonal response.4 And by week 15, the fetus is “extremely sensitive to painful stimuli” making it “necessary to apply adequate analgesia to prevent [fetal] suffering.”5 A more recent comprehensive scientific review of current literature concludes that preborn babies may experience pain as early as 12 weeks.6
Touch receptors first develop around the mouth around 7 ½ weeks7 and completely cover the body by 20 weeks;8 however, these early receptors only communicate touch information, not temperature or pain.9 Sensory fibers start forming synaptic connections with the spinal cord immediately,10 and participate in reflexively moving away from touches to the lips at 7 ½ weeks.11
The neurotransmitters dedicated to pain, such as substance P and enkephalin, appear at 10 to 12 weeks and 12 to 14 weeks respectively.12
Some scientists argue that pain, especially the emotional component, is not possible until the cortex is developed and connected to the sensory nerves in the body. These connections are found by 24 weeks.15 However, newer evidence suggests that the cognitive component of fetal pain can be processed using even earlier pathways in the cortical subplate between 12 and 24 weeks.16
Other studies have called into question whether adults need a cortex to process pain at all. For example, one case study has shown that a 55-year-old patient experienced pain even when he had extensive damage in the cortical regions that process pain.17 A second study examined the brains of two people who cannot feel pain and found that their pain-related brain areas were still active when they received multiple touches that healthy adults rated as painful, even though they could not feel the pain related to the touches.18 Together, these studies suggest that the cortex may not be necessary for pain perception at all.
Researchers have learned how the fetus experiences pain by comparing a painful procedure to a non-painful procedure in preborn children as early as 18 weeks.19 When researchers inserted a needle into the umbilical cord which does not have pain receptors, it did not appear to cause the fetus pain; however, when they inserted a needle into the vein near the liver to draw blood, it caused the fetus to react with “vigorous body and breathing movements” as seen by ultrasound.20 Furthermore, the fetus released stress hormones such as cortisol and noradrenaline.21 The fact that the circulating stress hormones in the bloodstream of the fetus increase without the same increase in the mother’s circulating stress hormones shows that the fetus perceives pain and reacts with a stress response at these early ages.
Furthermore, a 3D ultrasound recording of a fetus at 23 weeks showed a pained grimace on the fetus’s face immediately following an anesthetic injection before heart surgery in the womb.22 The same researchers recorded 13 fetuses at 31 weeks to show that the fetus responds with different facial expressions to painful versus startling stimuli.23 Researchers examined different facial gestures before and after a fetus heard a loud noise or received an anesthetic injection in their thigh. They found that the fetuses who received a painful injection made more grimacing and crying motions than those who were merely startled.24 These facial features are also associated with a conscious pain experience in newborn babies, who also cannot communicate their pain experience with words.
Watch the movie to see the fetus’s reaction to the painful injection.
Also of note, a number of compelling studies suggest that the fetus may actually be more sensitive to pain than full-term newborns and adults. The inhibitory serotonergic pathway, which blocks painful stimuli, only matures after birth.25 Premature infants born before 32 weeks gestation have a more sensitive limb-withdrawal reflex threshold than babies born after 39 ½ weeks.26 Additionally, when researchers studied babies born preterm, they found that a painful heel lance produced a response in the somatosensory cortex of babies at 25 weeks gestation.27 A different research group found that in premature infants born between 28 and 36 weeks, the younger the premature baby was, the greater their brain response to the painful heel lance.28 A number of compelling studies suggest that the fetus may actually be more sensitive to pain than full-term newborns and adults.