Aim: To clarify the significance of heart rate variability for the evaluation of an autonomic nervous system (ANS) in the normal fetus using fetal magnetocardiography (FMCG).

Methods: Subjects consisted of normal pregnancy (n = 35) at 28–39 weeks gestation. FMCG was recorded using 64-channel magnetocardiography (MCG) in a magnetically shielded room. The QRS interval was derived from signal-averaged MCG. The R–R interval variability induced by an R-wave trigger was eventually adopted to calculate for time-domain and frequency domain analysis. The power spectrum in the frequency domain was derived from frequency-field components using the maximum entropy method of fetal heart rate variability. Based on frequency analysis, the ranges of the LF and HF domains were defined as 0.01–0.15 and 0.15–0.4 Hz, respectively. We defined a coeffi cient of variance (CVRR) as an index of parasympathetic activity, and defined a low frequency/high frequency (LF/HF) ratio as a sympathetic activity.

Results: The value of CVRR in the normal pregnancy group displayed a slight increasing trend with gestational age (y = 1.77 + 0.10x; r = 0.32). In contrast, the LF/HF ratio in the normal pregnancy group clearly increased over the gestational period (one-way ANOVA: P = 0.003).

Conclusions: Analyses based on the time and frequency domains of heart rate variability using FMCG enable an evaluation of fetal ANS activity. Sympathetic nervous activity increased with gestational age in the normal pregnancy group.

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