The aim of this article is to investigate to what extent physical exercise and pregnancy affect each other. Despite an increase in metabolism at rest, the amount of oxygen available for exercise is virtually unaffected by pregnancy. The capability to perform exercise (in W) is equally unaffected, but body weight increases. As a result the maximal running speed and/or distance decreases during pregnancy. The fetus needs a constant supply of oxygen and nutrients. During exercise uterine blood flow decreases. This decrease is linearly correlated with the intensity and the duration of exercise and the maximal reduction averages approximately 25%. Nonetheless, uterine oxygen consumption is maintained. This is the result of compensation through hemoconcentration, redistribution of blood flow within the uterus, and increased oxygen extraction. A harmful effect of physical exercise on the child in utero, such as abortion, hypoxia, or growth retardation, has not been demonstrated so far. Although exercise does not seem to be harmful during normal gestation, one should be careful in case of pregnancy complications such as pregnancy-induced hypertension or premature contractions.