Aorta-caval compression due to maternal posture can change uterine artery blood flow, which is the major determinant of maternal intervillous perfusion and may affect MRI measures of placental oxygenation. We investigated the effect of maternal posture on estimates obtained from BOLD MRI of the placenta. We observed higher oxygenation signals in the supine position for a group at younger gestational age. In the group with higher gestational age, the influence of the maternal position on oxygen transport was inconsistent. These findings underscore the need to account for the effect of maternal posture on MRI studies of utero-placental circulation.