The question of when the human fetus develops the capacity for sentience is central to many contentious issues. The answer could and should influence attitudes toward IVF and embryo experimentation, abortion, and fetal and neonatal surgery. For the fetus to be described as sentient, the somatosensory pathways from the periphery to the primary somatosensory region of the cerebral cortex must be established and functional. Fetal behaviour is described and the development of the underlying anatomical substrate and the chemical and electrical pathways involved in the detection, transmission, and perception of somatosensory stimuli are reviewed.
It is concluded that the basic neuronal substrate required to transmit somatosensory information develops by mid-gestation (18 to 25 weeks), however, the functional capacity of the neural circuitry is limited by the immaturity of the system. Thus, 18 to 25 weeks is considered the earliest stage at which the lower boundary of sentience could be placed. At this stage of development, however, there is little evidence for the central processing of somatosensory information. Before 30 weeks gestational age, EEG activity is extremely limited and somatosensory evoked potentials are immature, lacking components which correlate with information processing within the cerebral cortex. Thus, 30 weeks is considered a more plausible stage of fetal development at which the lower boundary for sentience could be placed.
