Changes in fractal features of fetal heart rate during pregnancy

Our objective was to examine whether heart rate time series of healthy normal fetuses possess fractal properties and, if so, to determine whether consistent changes in fractal features according to gestational age exist.

One hundred nineteen fetal heart rate (FHR) recordings in 55 singleton pregnancies between the 22nd and 41st weeks were analyzed. Fractal analysis developed by Higuchi was performed. Changes of fractal dimension were examined according to gestational age.

Two characteristic scaling regions were present in each FHR trace. The fractal dimension defined within 500 ms to 5 s (D(S); median 1.396, range 1.273-1.642) was lower than that defined longer than 30 s (D(L); median 1.933, range 1.492-2.049) in every case. These two values were significantly different (p<0.001). There was a statistically significant difference in the values of D(S) between (22-29 weeks, mean 1.323), (30-33 weeks, mean 1.443) (p=0.004), (34-35 weeks, mean 1.418) (p=0.002), (36 weeks, mean 1.409) (p=0.030), (37-38 weeks, mean 1.394) (p=0.006), and (40-41 weeks, mean 1.452) (p=0.001), respectively. D(S) values between (40-41 weeks), (37-38 weeks) (p=0.012), and (39 weeks, mean 1.369) (p=0.030), respectively, were also significantly different. The values of D(L) decreased from 22-39 weeks (median 1.941) to 40-41 weeks (median 1.891) (p=0.008).

Two distinct fractal structures within the FHR variation were identified. Fractal features of heart rate of healthy normal fetuses change significantly during pregnancy period. Fractal analysis may be useful for evaluating FHR variation.