First, fetal EEG patterns and fetal behaviour demonstrate that sleep-like states of unconsciousness are continuously present throughout the last half of pregnancy (Mellor et al., 2005). This is because the continuous undifferentiated EEG patterns, which exhibit a mixture of REM-non-REM features, and the differentiated and alternating REM-non-REM patterns, which appear later and are indistinguishable from those seen during postnatal sleep, are all incompatible with consciousness. In addition, during labour there is a shift in the balance between the REM and non-REM states of unconsciousness such that the deeper non-REM state predominates (Mellor and Diesch, 2006).
Second, at least eight fetal, placental and uterine factors with well-demonstrated inhibitory effects on the fetal EEG apparently operate throughout the last half of pregnancy (Mellor and Gregory, 2003; Mellor et al., 2005), as summarised below... It is apparent, therefore, that mature fetal cerebrocortical function occurs in an inhibitory physiological environment which is unique to prenatal life.
Third, the neurologically mature fetus is not arousable from non-REM or REM sleep-like states to conscious wakefulness by potentially noxious interventions such as induced hypercapnia (high carbon dioxide), sounds loud enough to cause intense auditory pain and surgery-induced tissue damage (Mellor et al., 2005). This contrasts strikingly with the situation after birth where these potent stimuli do arouse the sleeping young to conscious wakefulness. This prenatal non-responsiveness to potentially noxious stimulation is a further indication of the unique inhibitory functional environment of the fetal brain. It also suggests that expulsion from the uterus at birth would lead to a marked reduction in overall neuroinhibitory influences on the brain. Indeed, this appears to be the case, because immediately after birth the major neuroinhibitors are substantially withdrawn and are replaced by a range of potent neuroactivators that support the onset of conscious awareness (Mellor and Diesch, 2006, 2007).
