Objective To test the null hypothesis that, after 28 weeks of gestation, uterine blood flow during supine rest and supine exercise is no different than uterine blood flow at left-lateral rest.Design In vivo experimental study in pregnant women.Setting Department of Obstetrics, MetroHealth Medical Center, Cleveland, OH, USA.Population Fourteen, physically active, late-pregnant women who continued supine exercise throughout gestation.Methods Studies were carried out between 29 and 38 weeks of gestation. Maternal blood pressure, maternal heart rate, and ultrasound estimates of volume blood flow in the right ascending branch of the uterine artery were obtained serially at rest in the left-lateral position, at rest in the supine position, during and immediately after 10 minutes of supine exercise, and again at rest in the left-lateral position. Exercise sessions included alternating 60-to 90-second periods of abdominal crunches and leg exercise at moderate/high intensity (Borg's rating of perceived exertion 14 ± 1).Main outcome measures Blood pressure, heart rate, and uterine artery volume flow.Results Data are presented as the mean ± SD. Maternal heart rate and blood pressure were unchanged at supine rest but increased during supine exercise (heart rate increased from 76 ± 9 to 98 ± 12 beats per minute, mean arterial pressure increased from 81 ± 6 to 102 ± 12 mmHg). Volume flow fell from 410 ± 93 to 267 ± 73 cc/ minute after 5 minutes of supine rest and then, during supine exercise, increased to 355 ± 125 cc/minute. Uterine artery luminal diameter and blood flow correlated directly with tissue weights at birth (r 2 values between 0.32 and 0.59).Conclusions In physically active women, uterine blood flow decreases during both supine rest and supine exercise but the decrease in the former is twice that seen in the latter.Keywords late pregnancy, supine exercise, supine rest, uterine blood flow Please cite this paper as: Jeffreys R, Stepanchak W, Lopez B, Hardis J, Clapp III J. Uterine blood flow during supine rest and exercise after 28 weeks of gestation.