Running Through Pregnancy: Cardiovascular and Training Adaptations in Recreational Endurance Runners

Background: Pregnancy represents a profound state of cardiovascular remodeling. Systemic vascular resistance decreases by up to 30%, compensated by a 40–50% increase in cardiac output and a shift in autonomic tone. An increase in resting heart-rate of 10-20 bpm can be usually seen by mid-pregnancy. Weight increases steadily beginning with a 0.5-2 kg gain in the first trimester. Contemporary guidance from the American College of Obstetricians and Gynecologists (ACOG) recommends at least 150 min/week of moderate-intensity aerobic activity throughout uncomplicated pregnancies. However, data on how recreational runners modulate their training during pregnancy remain sparse. Understanding these adaptations has relevance not only for optimizing maternal cardiovascular health but also for refining guidelines in gender-specific cardiology.
Objective: To quantify trimester-specific cardiovascular load, training adaptation, and biomechanical shifts in four recreational runners during pregnancy, using longitudinal, wearable-derived data. 

Methods: We analyzed 378 GPS-recorded running sessions from four healthy female recreational runners, spanning from March 2022 to May 2025. Each session was categorized into pre-pregnancy (Baseline), first (T1), second (T2), or third trimester (T3). One subject continued running through T3; two others had data up into T2 ; one ceased running in mid-T2 due to mechanical intolerance. Variables extracted from multisensor sports watches included distance, speed, mean/max heart rate, aerobic and anaerobic training load (Garmin™ physiology engine), cadence, stride length, vertical oscillation, and ground contact time. Averages were calculated per subject per trimester to characterize cardiovascular and mechanical trends.

Results: All four runners exhibited a marked reduction in anaerobic load from baseline to T2, with some subjects eliminating anaerobic sessions entirely. This indicates a shift towards predominantly oxidative work at intensities that demand smaller stroke-volume reserves and lower peripheral resistance. This mirrors laboratory data showing that the lactate threshold occurs at lower absolute workloads during pregnancy, with greater reliance on oxidative metabolism. Aerobic load declined moderately, suggesting preserved but re-regulated oxidative capacity. Mean heart rate during exercise decreased in all runners reflecting a self-limiting approach to exertion as stroke volume increased. This underscores the athletes’ down-regulation of relative workload to preserve cardiac output margin and avoid excessive sympathetic drive—an adjustment advocated by cardiovascular exercise position statements for pregnancy. Stride length decreased and ground contact time increased in all subjects, aligning with known changes in pelvic tilt and anterior mass distribution. All runners maintained or exceeded the ACOG-recommended 150 minutes of aerobic activity per week well into their second trimesters.

Conclusions: In this longitudinal case series, recreational runners modulated cardiovascular demand and mechanical strain in ways that mirrored physiological changes in pregnancy. The consistent drop in anaerobic load and exercise heart rate reflects a cardioprotective autoregulation of workload. These adaptations are relevant for clinicians working in sports and gender cardiology, supporting trimester-specific training strategies to maintain a safe cardiovascular condition during pregnancy.