For most of the 20th century, scientists denied that infants 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.
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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 is now standard procedure in prenatal surgeries.3
Touch receptors first develop around the mouth around 5 1/2 weeks after conception4 and completely cover the body by 18 weeks after conception;5 however, these early receptors only communicate touch information, not temperature or pain.6 Sensory fibers start forming synaptic connections with the spinal cord immediately,7 and participate in reflexively moving away from touches to the lips at 5 1/2 weeks after conception.8
The neurotransmitters dedicated to pain, such as substance P and enkephalin, appear at 8-10 weeks post-conception and 10-12 weeks post-conception respectively.9
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 22 weeks after conception.12 However, newer evidence suggests that the cognitive component of fetal pain can be processed using even earlier pathways in the cortical subplate between 10 and 22 weeks after conception.13
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.14 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.15 Together, these studies suggest that the cortex may not be necessary for pain perception at all.
Furthermore, researchers have learned about the fetal response to pain by comparing a painful procedure to a non-painful procedure in unborn children as early as 16 weeks after conception who did not receive anesthesia.16 When researchers inserted a needle into the umbilical cord 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”17 and release stress hormones such as cortisol and noradrenaline.18 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 at these early ages.
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.19 Premature infants born before 30 weeks post-conception age have a more sensitive limb-withdrawal reflex threshold than babies born after 37.5 weeks post-conception age.20 Additionally, when researchers studied babies born preterm, they found that a painful heel lance produced a response in the somatosensory cortex of babies at 23 weeks post-conception age.21 A different research group found that in premature infants born between 26 and 34 weeks post-conception age, the younger the premature baby was, the greater their brain response to the painful heel lance.22 A number of compelling studies suggest that the fetus may actually be more sensitive to pain than full-term newborns and adults.