Whether fetuses do indeed feel pain and, if they do, when do they acquire the ability to feel it are matters of great debate.

Fetal pain is a subject that is particularly prone to controversy, stretching far beyond its scientific aspects. And there is always increased attention to this topic whenever legislative changes regarding the intentional termination of pregnancy are impending.

Research on fetal pain is understandably complicated. The first and most obvious difficulty is that you cannot ask a fetus if something hurts and, therefore, there is never certainty that there is pain. When talking about pain, one must keep in mind its definition, which, according to the International Association for the Study of Pain, is: “An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.”

The emotional component of pain entails the need for consciousness to allow the recognition of the unpleasantness of a stimulus. It is this subjective cognitive component of pain that distinguishes it from nociception, which is defined as the neural processes of encoding actual or potentially tissue-damaging events.

Pain is a psychological state and therefore, when trying to infer whether fetuses can feel pain, not only the capacity to detect potentially painful stimuli, but also the capacity to consciously perceive them have to be evaluated. That possibility has to be inferred from data such as fetal anatomy, neurochemistry, behavior, the development of neural circuits, and the production of stress related hormones in response to stimuli. But even these aren’t easily studied.

In order for a stimulus to be perceived as painful, a whole circuitry has to be fully developed: information must travel from spinal cord neurons, whose axons project to the thalamus, which sends afferents to the cerebral cortex. From what research, within its limitations, has been able to determine, functional sensory fibers and spinal reflexes can be found by 20 weeks of gestation, as well as connections to the thalamus. However, this path becomes fully functional only in the third trimester, around 29 to 30 weeks’ gestational age, when mature projections from the thalamus to the cerebral cortex are present.

Although withdrawal reflexes in reaction to cutaneous stimulation are observable at earlier developmental stages, these do not imply effective pain perception since they are not specific to nociceptive stimuli and are not dependent on cortical mediation. Likewise, stress responses such as increased blood flow, heart rate and respiratory rate, as well as neuroendocrine changes that include increased production of catecholamines, cortisol, and other stress hormones, or increased beta-endorphin or noradrenaline release, can also be observed early in the second trimester, but again, these are not necessarily indicative of pain perception. 

Overall, amidst the debate, with some arguing that fetusus can feel pain earlier by using subcortical structures, and others arguing that the fetus cannot feel pain by being maintained in a state of sedation in the womb, it is generally agreed that the minimal necessary neuronal pathways for pain are in place by 24 weeks gestation.

References

Bellieni CV, & Buonocore G (2012). Is fetal pain a real evidence? The journal of maternal-fetal & neonatal medicine : the official journal of the European Association of Perinatal Medicine, the Federation of Asia and Oceania Perinatal Societies, the International Society of Perinatal Obstetricians, 25 (8), 1203-8 PMID: 22023261

Derbyshire SW (2010). Foetal pain? Best practice & research. Clinical obstetrics & gynaecology, 24 (5), 647-55 PMID: 20356798

Lee SJ, Ralston HJ, Drey EA, Partridge JC, & Rosen MA (2005). Fetal pain: a systematic multidisciplinary review of the evidence. JAMA : the journal of the American Medical Association, 294 (8), 947-54 PMID: 16118385

Lowery CL, Hardman MP, Manning N, Hall RW, Anand KJ, & Clancy B (2007). Neurodevelopmental changes of fetal pain. Seminars in perinatology, 31 (5), 275-82 PMID: 17905181

Rokyta R (2008). Fetal pain. Neuro endocrinology letters, 29 (6), 807-14 PMID: 19112406

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