Attenuation of short-term autonomic cardiovascular responses with advancing gestational age

Pregnancy imposes significant physiological stress, requiring substantial cardiovascular adaptation. While it is well-known that the autonomic nervous system (ANS) shifts toward a relative sympathetic dominance, the longitudinal trajectory of autonomic flexibility—specifically the ability to modulate and respond to stressors across all three trimesters—remains poorly defined. This study aimed to characterize these longitudinal changes in ANS regulation and reactivity in healthy pregnant women.

Researchers conducted a prospective longitudinal study of 67 healthy pregnant women evaluated during the first (T1), second (T2), and third (T3) trimesters. Using continuous monitoring, they assessed cardiovascular parameters during a standardized, multi-phase protocol consisting of resting periods, paced breathing, and a cognitive memory task. The study analyzed heart rate variability (HRV) indices, blood pressure, and phase synchronization measures to quantify autonomic regulation, reactivity, and cardio-respiratory coupling.

Results indicated that while the maternal body maintains the ability to shift heart rate and blood pressure in response to stimuli, the magnitude of these responses diminishes as pregnancy progresses. Vagal-related HRV parameters, such as RMSSD, declined by the third trimester, and the cardio-respiratory synchronization indices (which reflect the alignment of heart rate and breathing rhythms) were significantly lower during paced breathing at T3. Although the sympathetic and parasympathetic branches remained functionally responsive, the study identified a consistent "blunting" or narrowing of the dynamic range of autonomic reactivity, suggesting a progressive reduction in autonomic flexibility toward the end of pregnancy.

The study provides a normative physiological framework for autonomic adaptation during healthy pregnancy. By moving beyond resting-state metrics to examine task-related autonomic reactivity, the authors identify a narrowing of the regulatory reserve that characterizes late gestation. This insight is critical for future research into pregnancy-associated cardiovascular complications, such as preeclampsia or gestational diabetes, where early deviations from these normal autonomic trajectories could serve as valuable clinical biomarkers for identifying women at higher risk.