Attenuation of short-term autonomic cardiovascular responses with advancing gestational age
Introduction Pregnancy induces profound cardiovascular adaptations, including changes in vascular resistance, cardiac output, and heart rate, which are partly regulated by the autonomic nervous system. Although a shift toward reduced parasympathetic activity has been reported, the longitudinal evolution of autonomic function and its responsiveness to physiological stressors across gestation remain incompletely defined. This study aimed to characterize trimester-specific changes in autonomic regulation and reactivity. Methods In this prospective longitudinal cohort, 67 healthy pregnant women were evaluated during the first (T1), second (T2), and third (T3) trimesters. Continuous cardiovascular monitoring was performed during a standardized protocol including rest, paced breathing, and a cognitive task. Heart rate variability, blood pressure, and phase synchronization indices were analysed using two-way repeated-measures ANOVA (Time × Phase). Results Heart rate increased across gestation, while blood pressure showed a mid-pregnancy nadir. Indices of vagal modulation declined from T1 to T3, whereas overall variability remained unchanged. No effect of gestational age was observed for sympathovagal balance, but significant Time × Phase interactions indicated attenuation of autonomic reactivity. Both vagal responses to paced breathing and sympathetic responses to cognitive stress were reduced in late pregnancy. Similarly, indices of cardio-respiratory and cardiovascular coupling showed phase-dependent changes, with reduced synchronization during paced breathing in T3. Discussion Our findings suggest that autonomic regulation during pregnancy is characterized by preserved responsiveness but reduced modulation. The decline in vagal activity, together with attenuated reactivity and reduced cardio-respiratory coupling, suggests reduced autonomic flexibility. These findings outline autonomic trajectories in healthy pregnancy that may serve as a normative reference against which early deviations could be examined in future studies on pregnancy-associated cardiovascular complications.