Women who fly during pregnancy, whether as passengers or crew, continue to fuel the debate over the potential impact on pregnancy outcome and fetal development, the two risk factors most commonly invoked being relative hypoxaemia due to the decreased cabin pressure and, more recently, cosmic radiation. On both theoretical and experimental grounds (altitude physiology and studies in pregnant women during flight), commercial flight poses no threat to the fetal oxygen supply in a normal pregnancy. As for cosmic radiation, only theoretical estimates are available of flight crew exposure: if annual doses approximate to background at ground level (3-5 mSv), the dose received during an individual pregnancy can be estimated from the fraction of annual flight time spent while pregnant. It is doubtful whether any epidemiological study could ever confirm or refute this theoretical estimate of a low increase in risk. Many airlines have opted to allow pregnant crew to continue flying. There is thus little if any ground for advising against passenger flight in pregnancy. Flying is probably the safest and most comfortable way to travel long-distance in pregnancy. The few relative contraindications include flying close to term, a history of miscarriage and premature delivery, heavy smoking, severe anemia, cardiopulmonary disease, and a serious fear of flying.