Alteration in the Low-Frequency Domain in Power Spectral Analysisof Fetal Heart Beat Fluctuations

<i>Objective:</i> Spectral analysis was conducted on fetal heart rate fluctuations for the purpose of investigating the change in gestational age and examining the usefulness as a method of estimating fetal blood gas. <i>Study Design:</i> Five hundred and twenty-nine pregnant women with a single normal fetus and 26 women with intrauterine growth restriction (IUGR) were studied during the 20th to 38th gestational weeks. In cases of IUGR, fetal blood gas levels were evaluated with percutaneous umbilical blood sampling. A time series of fetal heart rate fluctuation was obtained with a 1.15-MHz ultrasonic transducer, and power spectral analysis of 200 consecutive stable beats was performed with an autoregression method. Integrated areas of 0.025–0.125 cycles/beat were defined as low-frequency areas (LFAs) and were examined in normal fetuses. In addition, the relation between LFA and fetal blood gas values was studied. <i>Results:</i> As pregnancy progressed, LFA increased with a cubic regression equation (Y = 6.484 – 0.764X + 0.029X² – 0.00034X³). The correlation coefficient was 0.625 (p < 0.05). In the cases of IUGR, ΔLFA was highly correlated with ΔpO₂ and ΔpH (correlation coefficient, 0.650 and 0.618, respectively). <i>Conclusions:</i> Measurement of LFA provides insight into the development of fetal autonomic function and also may serve as a quantitative index of fetal well-being in mid-pregnancy.