Analysis of term and placental influences on the relative energy spectrum of uterine electrical signals. (a) Relative energy spectrum (RES) computed from pregnant monkey EMGs recorded during the last third of pregnancy and labour. Notice the shift towards higher frequencies as pregnancy progresses. (b) RES computed from pregnant monkey EMG's recorded during pregnancy, with two electrode positions: electrodes away from the site of placenta insertion (No Placenta); electrodes located above the placental insertion (Placenta) (**p < 0.01). AU, arbitrary units.