Traditional medical advice to pregnant women in the U.S. has discouraged exercise. This restrictive attitude has denied many women an essential healthful behavior at an important period of their lives. Regular physical activity is a behavior to which the human body has adapted over millions of years of evolution. It is a behavior that is as vital, safe, and natural during pregnancy as at any other time. We maintain that healthy women with uncomplicated pregnancies can perform regular upright moderate intensity exercise (50-65% VO2 max) as well as upright vigorous exercise (65-85% VO2max), without endangering themselves or compromising fetal well-being. We further maintain that they should be encouraged to do so regularly. Exercise prescription should be individualized, and not based on arbitrary standards.