Physiological basis for recommendations regarding exercise during pregnancy at high altitude.

Although exercise during pregnancy has been shown to be safe and is now recommended by the American College of Obstetricians and Gynecologists and the Society of Obstetricians and Gynaecologists of Canada for all healthy women, little is known about the combined effects of high altitude and exercise during pregnancy. Due to a paucity of experimental data, recommendations regarding exercise during pregnancy at elevations >/=1600 m must be based on synthesis of available information regarding the independent effects of high altitude and exercise on uteroplacental oxygen delivery. Maternal hyperventilation and elevated blood hemoglobin concentration maintain resting arterial oxygen content at or above sea-level values. During exercise, however, arterial hemoglobin saturation falls, lowering the oxygen content. Resting uterine artery blood flow is lower in residents at 3100 m than at 1600 m, and flow is likely to decrease further during exercise, in proportion to the intensity and duration. The combined reduction in arterial oxygen content and uterine artery flow suggest that fetal oxygen delivery is compromised during exercise at altitude. Anemia, which should be defined according to altitude-adjusted criteria, will further diminish oxygen delivery. Clinicians may wish to monitor patient hemoglobin concentration more frequently and adopt a more conservative stance regarding exercise during pregnancy for both residents and visitors of altitudes >/=1600 m.