||PURPOSE: Our previous research found lower fetal heart rates (HR) and increased heart rate variability (HRV) in women who exercised during pregnancy. This finding is similar to the adult heart training response at rest due to aerobic exercise. Dose response associations have been found between physical activity and cardiorespiratory fitness in adults. Therefore, our objective was to determine if there is a dose-response relationship between maternal physical activity and fetal HR, HRV, and sympatho-vagal balance. METHODS: Pregnant women completed a physical activity questionnaire and magnetocardiogram (magnetic correlate to electrocardiogram) recordings at 36 weeks gestational age. Women reported duration, intensity, and frequency of each activity for each month of pregnancy as well as 3 months prior to pregnancy. These values were used to calculate maternal physical activity measures for each participant. Relationships between fetal HR, HRV, and sympatho-vagal balance at 36 weeks gestational age and maternal physical activity (n=50 pairs) during the 3 trimester were assessed by Spearman correlations. Regression analysis was performed to further examine these relationships after controlling for maternal and fetal covariates (maternal age, maternal resting HR, maternal weight gain, pre-pregnant body mass index (BMI), and fetal activity state) RESULTS: The regression analyses showed that maternal physical activity intensity (kcal·min) was negatively associated with HR in the active fetal state (p<0.05) and physical activity duration (min/3 trimester) was positively associated with fetal HRV (p<0.05). There were no statistically significant relationships with maternal physical activity on measures of fetal sympatho-vagal balance. CONCLUSION: Maternal physical activity dose during the third trimester is associated with resting fetal heart effects similar to a trained response. Future studies on the health benefits of this fetal response are highly warranted.