Electrocortical activity in fetal sheep in the last seven days of gestation

1 Electrocorticogram (ECoG) and myometrial electromyogram (EMG) were recorded continuously in chronically instrumented late gestation sheep fetuses ( n = 9 ) to analyse: (1) 24 h ECoG patterns, (2) relationships between ECoG and myometrial contractility, and (3) 24 h ECoG patterns at the spontaneous onset of labour. 2 Spontaneous onset of labour was determined from the myometrial EMG. ECoG signals were purified by a denoising procedure of wavelet decomposition. High‐voltage slow ECoG activity (HV) and low‐voltage fast ECoG activity (LV) were determined mathematically, and HV‐LV cycle periodicity was calculated by periodogram analysis. 3 Twenty‐four hour rhythms were present in fetal ECoG HV‐LV cycles in the 3–5 days prior to spontaneous onset of labour ( P < 0.01 ). Cycle frequency was lower at 08.00–12.00 h and higher at 16.00–20.00 h (lights on at 07.00 h, lights off at 21.00 h). LV duration was longer at 20.00–24.00 h, and HV was shorter at 16.00–20.00 h. No relationship was found between ECoG cycles and myometrial contracture cycles. Twenty‐four hour ECoG rhythms disappeared 1 day before the spontaneous onset of labour. 4 ECoG patterns changed 7 to 4 h prior to spontaneous onset of labour, percentage of time spent and duration of HV ECoG increased, and percentage of time spent in LV decreased significantly. ECoG HV‐LV cyclicity was reduced 4–5 h prior to spontaneous onset of labour, indicating that the altered fetal hormonal and blood gas environment around the spontaneous onset of labour alter fetal neural function.