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Aims: Heart rate variability (HRV) reveals information on the functional state of the autonomic nervous system (ANS). This study was initiated to assess the physiological and maturational development of the ANS by comparing HRV data of a large number healthy foetus of different gestational ages from the 24(th) to the 42(nd) week., Methods: Cardiotocogram (CTG) recordings of HRV of 172 healthy foetus (24-42 weeks' gestation) were performed to establish normative data. Frequency domain HRV parameters were computed in three frequency bands., Results: The gestational ages of the foetuses correlate with HRV. Lower gestational age in weeks showed lower frequency domain parameters than higher gestational age in weeks. The most significant differences were discerned for HRV parameters reflecting sympathetic activity in LF (low frequency) and VLF (very low frequency), due to the adrenergic system., Conclusions: Maturation of the ANS is accompanied by increasing HRV with a pronounced increase of sympathetic activity. These changes are measurable by CTG recordings with a computer algorithm which can calculate short-term variability on the basis of CTG data. Problems of the CTG signal, as shown before, were the parameters of the time domain, which could only be well detected by ECG or foetal magnetocardiography (FMCG)., (Copyright Georg Thieme Verlag KG Stuttgart, New York.)