Chaos and fractals which 1/f spectrum below 10(-2) Hz demonstrates full-term fetal heart rate changes during active phase.

Human fetal heart rate changes were analysed using power spectral density (PSD). The data for analysis were obtained from 25 normal fetuses between the 37th and 40th week of gestation. Pulse interval distribution showed an almost Gaussian distribution. PSD was calculated by first Fourier transform technique, and showed the particular fluctuation below 10(-2) Hz to be inversely proportional to frequency (so called '1/f spectrum') during the active phase after the 37th week of gestation. These results revealed that the 1/f spectrum of the human heart rate had already appeared below 10(-2) Hz in full term of pregnancy.