However, because the air pressure in the cabin is less than at lower altitudes, your heart rate and blood pressure will increase to enable you to take in the oxygen you need. If you have severe anemia, sickle cell disease, a history of blood clots, or a condition called placental insufficiency, you and your baby may have trouble adapting and should avoid flying. If you have one of these conditions and must fly, you can be prescribed supplemental oxygen for use in the air. (And in case of sudden loss of cabin pressure during a flight, all commercial airliners are equipped with oxygen masks that drop down automatically.)
Flying in unpressurized small planes is a different matter. If you're cruising at 10,000 feet, for example, that's just like standing atop a 10,000-foot mountain — almost 2 miles high. Your body will have to work harder to supply you and your baby with sufficient oxygen, so it's probably wise to avoid unpressurized planes while you're pregnant.