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Objective: The aim of this study was to examine changes in the heart rate variability based on the frequency power spectrum of healthy fetuses during the second and third trimester of pregnancy., Methods: We analyzed 222 fetal magnetocardiograms recorded in 49 healthy singleton pregnancies between the 16th and 42nd week. Discrete Fourier transformation was performed on the time-based step function of the RR-intervals. Changes of spectral density in the frequency spectrum in various bands between 0.003 to 1 Hz, including low-frequency (LF: 0.04-0.15 Hz) and high-frequency (HF: 0.15-0.40 Hz) bands, were examined as a function of gestational age., Results: Spectral density between 0.003 to 1.0 Hz increased with gestational age with large changes, in particular, at lower frequencies. At approximately the 32nd week, the rate of increase in power slowed substantially. Prior to this time, the rates of change in power were different for the bands 0.003 to 0.40 Hz, 0.40 to 0.60 Hz and 0.60 to 1.0 Hz. LF and HF showed similar development, with HF increasing slightly more rapidly., Conclusion: We conclude that characteristic spectral bands that increase in spectral density at different rates during the second and third trimester may be identified. They most likely reflect developmental changes and behavioral states during pregnancy., (Copyright 2003 John Wiley & Sons, Ltd.)