The dual stresses of pregnancy and exercise may create conflicting physiological demands that could adversely affect pregnancy outcome. Specifically, redistribution of uterine blood flow and subsequent fetal hypoxia, hyperthermia and the risk of teratogenic effects, decreased carbohydrate availability for the fetus, and increased uterine contractility with a possible increase in risk for pre-term labour, all pose potential threats to fetal growth and development. However, despite these potential risks, literature dealing with exercise and pregnancy outcome generally shows neutral or somewhat favourable effects. A few studies have found reduced birthweight, shortened gestation, and less gestational weight gain among women who continue vigorous exercise during pregnancy compared with those who discontinue exercise or who are sedentary. However, most studies find little, if any, association between exercise and birthweight or gestational age. In contrast, studies of occupational physical activity often show an association between heavy physical work and lower birthweight and shorter gestation, especially in women in developing societies whose nutritional status may be compromised. Standing, in particular, may be associated with increased risk of prematurity. Although other outcomes, such as length of labour, type of delivery have not been well studied, there is no indication of any negative associations with exercise. There is limited evidence which suggests that exercise is related to shorter labour and is a useful treatment for gestational diabetes. Exercise is also associated with fewer symptoms and discomforts of pregnancy. This relationship is temporal in that exercise earlier in pregnancy is associated with fewer symptoms later in pregnancy. The lack of evidence for any harmful effects of exercise on pregnancy outcome indicates that, for healthy, well-nourished women, exercise during pregnancy is safe and subject to few restrictions. This conclusion is reflected in the revised, 1994 recommendations of the American College of Obstetricians and Gynecologists.