Fetal autonomic responses inferred from heart rate variability during recurrent decelerations without acidosis—An observational study

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Abstract
Objective: Recurrent fetal heart rate (FHR) decelerations without acidosis are frequently observed during labor; however, the underlying autonomic mechanisms remain incompletely understood. This study explored fetal autonomic responses to recurrent decelerations using FHR and FHR variability (FHRV) parameters. Methods: This retrospective observational study included healthy women with singleton pregnancies, no pregnancy complications requiring treatment, and an umbilical artery pH >7.2 at delivery. Continuous FHR monitoring using fetal head electrodes was performed for ≥2 h before delivery (June–December 2021). FHRV was assessed using time‐domain, frequency‐domain, and nonlinear indices. Results: Twenty‐four patients were included. Baseline FHR did not differ between groups. In the recurrent deceleration group, root mean square of successive differences and high‐frequency power were higher during the final 60 min before delivery. No differences were observed in standard deviation of normal‐to‐normal intervals or very‐low‐frequency power. Sample entropy was also higher, suggesting increased complexity of autonomic regulation. Conclusion: Fetuses with recurrent decelerations without acidosis demonstrated patterns consistent with increased vagal modulation, as inferred from FHRV indices. These findings may reflect an adaptive autonomic response; however, given the exploratory nature and small sample size, further studies are warranted.

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