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Introduction: Self-reported maternal exercise is associated with improved heart and body size measures in the fetus. Currently, there is a gap in understanding how regular maternal aerobic exercise at recommended levels (150 min/wk) for pregnancy influence heart and body size in utero.Hypothesis: We assessed the hypothesis that offspring of women randomized to aerobic exercise will have lower heart rate (HR), higher ejection fractions (EF), and lighter fetal weight than offspring of women in the control group.Methods: Healthy pregnant women were randomly assigned to aerobic exercise or no intervention. Women performed aerobic activity at a moderate intensity for 150 min/wk from 16 weeks until delivery. Maternal demographics (age, pre-pregnancy BMI) as well as other activities were self-reported. Based on type and intensity, activities were converted to METs and then combined to yield METmin per week (METs X frequency per week X duration in minutes). A sonographer blinded to group assignment recorded fetal ultrasound. Data from 27 pregnancies (16 exercise, 11 control) were analyzed. To compare differences between groups (Active only=A; Quiet only=Q) independent t-tests were performed while regression analysis was conducted to determine predictors of fetal HR, our primary outcome.Results: Of the 16 exercisers, 6 were excluded (compliance 51-74%); the remaining 10 had >80% compliance. There were no group differences in maternal demographics, i.e., age and pre-pregnancy BMI. All values were in the normal range for fetal heart and body size measures. While not statistically significant, there was trend for fetal HR to be lower in exercisers (p=0.14) while aortic (p=0.05) and pulmonary (p=0.22) diameter, right ventricular (RV) EF (p=0.17), and left ventricular (LV) EF (p=0.18) were slightly higher compared to controls. In the A state, the female HR of exercisers were lower (p=0.05), while male left ventricular (LV) EF (p=0.006), and all fetuses right ventricular (RV) EF (p=0.07) were increased relative to same gender controls. In the Q state, female aortic diameter (p=0.11) and all fetuses’ pulmonary diameter (p=0.04) were increased in exercisers relative to same gender controls. Exercise level during pregnancy was a significant predictor of fetal HR (R2=0.10; p=0.01). Fetal head to abdominal circumference ratio was appropriate in exercisers (1.0±0.1) relative to control (0.985±0.1) fetuses (p=0.09). The fetuses of exercisers were slightly lighter relative to fetuses of controls (p=0.20).Conclusion: These preliminary data support the positive influence of aerobic exercise at recommended levels on fetal health outcomes during pregnancy.