Advice to pregnant women wishing to exercise at altitude is based only on isolated observations and a handful of systematic studies, none of which have explored the limits of combined exercise and altitude exposure in human pregnancy (ie, at maximal exercise and maximal altitude). Guidelines must therefore be based on a variety of sources, including the physiology of permanent residence at high altitude, in particular the degree of (and hence requirement for) adaptations specific to pregnancy over and above simple altitude acclimatization, and data from the few systematic studies in human pregnancy under conditions of short-term altitude exposure with or without exercise. Because the effects of altitude and exercise may be synergistic rather than additive, and because individual altitude tolerance and exercise capacity cannot be reliably determined at sea level, advice should err on the side of caution by not allowing for the substantial safety margins by which pregnancies are probably protected. An altitude of 2,500 m (8,250 ft) should not be exceeded in the first 4 to 5 days of short-term exposure. If exercise is performed directly after exposure, this should take place at correspondingly lower altitude, especially in the first few days. Compounding risks, eg, maternal smoking, anemia or fetal growth retardation, must be carefully excluded.