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

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.