IntroductionIn the field of maternal fetal medicine, animal experiments such as the chronic preparation model have been used for many years and provided a substantial amount of knowledge about fetal physiology 1）2） . The chronic preparation model is a wellestablished method of studying fetal physiology and pathology；however, its use means that fetuses cannot be directly affected by researchers, and thus can only be affected indirectly through their mother. Furthermore, responses from the fetus that are not influenced by the mother cannot be Abstract 1．OBJECTIVESTo investigate the physiological states of goat fetuses undergoing extrauterine incubation following isolation from their mother, we analyzed fetal electroencephalograms（EEGs） with power spectral analysis. 2．MATERIALS AND METHODSTen goat fetuses at 124 ± 3.9 days of gestation（term 150 days） were incubated in an artificial womb connected to an extracorporeal membrane oxygenation（ECMO） apparatus. While these fetuses were in a stabilized condition, their EEGs were recorded using a digital data recorder.Electrocortical activity on the EEGs was classified visually as either a high-voltage period or a low-voltage period. The recorded data were then analyzed using a computer. After analogue-todigital conversion at a sampling frequency of 300 Hz with a resolution of 16 bits, power spectral analysis using an auto-regression model was carried out. 3．RESULTSBoth high-voltage and low-voltage periods could be observed on the EEGs of all ten goat fetuses, and these periods appeared to alternate. Power spectral analysis of recorded EEGs in the high-voltage period showed a peak at the 0.5─4 Hz frequency range, which corresponds to the delta band. A peak at the 8─13 Hz frequency range, which corresponds to the alpha band, was observed in the low-voltage period. 4．CONCLUSIONDuring extrauterine incubation, goat fetuses showed two different EEG patterns. This suggests that the fetuses show physiological behavioral states that are independent from those of their mother.