Pregnant women spend more than half of the day in an upright position. The physiological effects of this posture on the mother and the fetus are evaluated. Changes in vascular autoregulation and anatomy lead to maternal fainting in about 8% of women during early pregnancy. The immediate effects of such episodes on the fetus are unknown. There is a positive correlation of orthostatic dysregulations and abortions. In late pregnancy we found a significant increase in functional residual capacity in the upright posture. Minute volume and oxygen consumption were also significantly increase (p less than 0.001). Regarding the cardiovascular changes we detected a rhythmic change of the maternal heart rate with the change to upright position, which had not been published before. Change from the left lateral position to unsupported standing increased maternal heart rate by a mean of 27 beats per minute and a mean duration of 105 seconds in two thirds of the women. This was accompanied by a decreased cardiac output, systolic blood pressure and an increased oxygen consumption. The gravid uterus is responsible for these changes. During the upright position, the venous flow to the right ventricle is inhibited by the relaxed uterus. Contractions, leaning forward and the muscle pump improve the venous return. The phenomenon reached its maximum during the 38th week, where 71% of pregnant women displayed a cyclic change in heart rate. The fetal heart rate baseline is significantly increased in the upright position with a significantly reduced acceleration frequency (p less than 0.001). Combined with the data from epidemiologic studies, prolonged standing during late pregnancy may signal potential risks for the fetus such as low birth weight, prematurity and stillbirths because of an 'uterovascular syndrome'. Maternal standing possibly may be used as a physiological fetal stress test.