In the past decade, physical health fitness has become increasingly more important in the lives of women of child-bearing age. Many have made regular, and sometimes vigorous commitments to exercise programs. In 1985, the American College of Obstetricians and Gynecologists (ACOG) developed a set of guidelines for women who plan to exercise during pregnancy. Recommendations were based on available, but somewhat limited, scientific data and common sense. Since that time, researchers have learned a significant amount of new information about how a pregnant woman and her fetus respond to aerobic activity. The objective of this communication is to review recent investigations in this area. Specific topics include a) maternal responses to exercise, b) fetal responses to maternal exercise, c) animal research models, and d) pregnancy and physical conditioning. Our objectives are to present information that will a) stimulate new and innovative research designs for exercise and pregnancy studies, and b) add significantly to our knowledge and ability to develop safe and effective exercise programs for women who wish to remain physically active throughout a normal-term pregnancy.