The paper presents the well-established spectral analysis approach to diagnose the physiological state of an unborn using phonocardiography. The separation of systole and diastole segments from a complete cardiac cycle of the fetus is carried out through envelope detection and threshold decision. The averaged periodogram is used for estimating both the spectral distribution and the dominant peak of the fetal heart sound signal. The current study clearly demonstrates that the fetal heart sound signals and their frequency spectra from the pathological subjects exhibit markedly different characteristics from those of healthy subjects. It can be safely concluded that the frequency spectrum and its contents of fetal cardiac sounds are specifically related to the health of the fetus. The importance of this study lies in the fact that with this approach, recognition and accurate estimation of systole and diastole periods of the fPCG signal as well as the quantification of their frequency contents can be performed very efficiently. Copyright © 2012 Inderscience Enterprises Ltd.