Cardiotocography (simultaneous recording of fetal heart rate (FHR) and uterine contractions) is one of the most used diagnostic techniques to evaluate fetal well-being and to investigate the functional state of the fetal autonomic nervous system. Recently, great interest has been paid to the variability of the FHR, and its frequency analysis, as a base for a more objective analysis of the cardiotocographic (CTG) tracings. FHR signals are unevenly sampled series. To obtain evenly sampled series, cardiotocographs often use zero-order interpolation. Such process is simple and fast but results unsuitable for frequency analyses because it introduces alterations in the FHR power spectrum. An algorithm for the recovery of the true FHR series out of the zero-order interpolated CTG data was developed and evaluated.