PURPOSE: The purposes of this study were to determine the normal values of spectral and chaotic indexes and to assess the maturational process of autonomic and chaotic control of fetal heart rate according to gestational age. METHODS: Three hundred and thirty-seven mothers with normal singleton pregnancies, who were 23 weeks or more in gestational age, was randomly chosen. For more than 40 minutes fetal heart rates and fetal movements were recorded and were downloaded to a computer. From each heart rate data, we extracted 25 minutes (3,000 points) worth of time series of dimension were calculated and then 2 weeks' average were compared to find the maturational change of them. RESULTS: LF power and LF/HF ratio which are indexes of cardiac sympathetic tone increased significantly (P=0.0001) with gestational age from 23 to 38 weeks. The HF power did not show significant developmental change. Correlation dimension, an index of chaotic control, tended to decrease with the gestational age (P=0.0001). It was 4.07 +/- 0.13 before 24 weeks and, after it reached the peak of 4.15 +/- 0.14 at 31-32 weeks, it gradually decrease to a slight but significant lower value of 3.67 +/- 0.08 at 41-42 weeks. CONCLUSION: Magnitude of cardiac sympathetic modulation increases until 37-38 weeks. On the average, fetal heart rate is regulated at least by four independent variables throughout the gestation and after 23 weeks of gestation. Complexity of heart rate control system peaks at 31-32 weeks then slightly decreases until term gestation, probably to synchronize the heart rate to other physiologic functions.