Flying with 400 other people, inside an enclosed cabin, 12,000 meters above the ground is an experience that has been available to humans only for a little more than a century. It is not a normal environment. In fact, it would be hostile, but for a string of protective measures that have been implemented since 1903, when Orville Wright lifted off the ground for the first powered flight, lasting just 12 seconds. I use the word hostile, because the environment outside the aircraft is hostile to human physiology. You cannot survive long at 12,000 meters, because it is extremely cold, and because the atmospheric pressure at such an altitude is only about one sixth of the pressure at sea level. As a consequence, the partial pressure of oxygen (O2) –the amount of pressure, due particularly to the O2 rather than to all the gases in the air– at such an altitude is too low to keep the O2 pressure in your lungs and blood high enough to supply your body tissues with adequate oxygen. This is one reason why aircraft cabins are pressurized, meaning that air pressure inside the cabin is elevated compared with the outside air. Another reason why aircraft cabins are pressurized is to reduce the risk that crews and passengers will develop what’s called decompression sickness (DCS), a condition related to nitrogen forming bubbles in body tissues, rather than remaining dissolved.
Something that is important to consider, however, particularly if you are pregnant, when we say that an aircraft cabin is pressurized, is that this usually doesn’t mean that it is pressurized to the same atmospheric pressure that we experience on Earth at sea level. Only very special aircrafts, certain medical evacuation aircrafts, provide sea level pressure. Ordinary passenger aircrafts provide something less than sea-level pressure. In a Boeing 767 aircraft, for example, cabin pressure at cruising altitude, is typically the equivalent of about 2,100 meters (7,000 ft); in other words, it’s like being in an aircraft that is not pressurized but is flying only at 2,100 meters altitude. This protects you from DCS, so long as you haven’t been SCUBA diving within a day or two before takeoff. It also makes the O2 partial pressure high enough, so that most passengers do not develop hypoxia, which means low O2 levels in body tissues. For some people, though, even the slightly lower O2 level aboard an aircraft could be problematic, namely in the case of somebody with a respiratory condition, such as chronic obstructive pulmonary disease (COPD). When it comes to the health of the fetus, this is the “$64,000 question.” Generally, you can provide enough oxygen to your fetus, because the fetus actually has hemoglobin in his/her red blood cells that can acquire good amounts of O2 from the mother. Nevertheless, if you are traveling long distances, if you have a wait on the ground, or have a connection, and if there is plenty of time, going outside to get some fresh air is a good idea.
Another factor that sometimes concerns pregnant women is the increase in ionizing radiation to which flyers are exposed. On the ground, we are well protected from radiation coming from outer space, but we do get some exposure. That’s called “background radiation”, which is a combination of radiation coming from the ground and radiation coming from space. When you fly, if you are at high latitudes on a typical transoceanic flight, you will receive an increased dose of ionizing radiation equivalent to about a chest x-ray or two worth of radiation per hour. In other words, if you take a ten-hour flight, it would be as if you had received about ten diagnostic X-ray scans. This is perfectly fine. It is not thought to be dangerous to the fetus. Furthermore, it should give you a perspective on getting medical radiation while pregnant. Namely, if you are not worried about radiation while flying during pregnancy, you certainly should not worry about a chest x-ray, or a dental x-ray scan, or an x-ray scan to check for a fracture, if needed while you are pregnant.
What should be a concern, however, particularly during the final trimester, is the risk of developing what doctors call a deep venous thrombosis (DVT). This is a blood clot in a deep vein, usually in the leg or pelvis. During late pregnancy, in preparation for blood loss during delivery, and influenced by hormonal changes, a woman’s blood clotting system is altered in a way that makes it a little easier to form clots than it is normally. This can help stop bleeding, but it also can cause blood clots obstructing a blood vessel. This extra tendency to form clots extends beyond the delivery of your child into the breast-feeding period. Outside of pregnancy, women also have an extra tendency toward DVT, if they take oral contraceptives (combined oral contraceptives as opposed to the mini-pill), or if they smoke. But you should not be smoking for a plethora of other reasons.
On top of these changes, if you are cramped into a seat for more than an hour or two, without moving your legs, the chances of developing a DVT increase still more. Along with the potential to cause pain and swelling in your leg, a DVT can release pieces of clots that travel through the blood stream as emboli. An embolus, a traveling clot, can then obstruct a large blood vessel in the lung. This is called a pulmonary embolism (PE) and it is extremely dangerous. It is life threatening and requires immediate therapy, which is very difficult on an aircraft, so the best way to deal with the problem is to prevent it. You can prevent a PE by preventing a DVT. This you can do by standing up, and moving your legs, as much as the flight crew allows you. Whenever possible, walk up and down the aisle, find some space, shake your legs and do knee bends. And when you must remain in your seat for long periods of time, try doing some leg exercises, such as extending and flexing your feet, and moving your legs within the cramped space. Of all the possible dangers on a long flight, the risk of DVT is the one risk that you should take seriously when flying while pregnant, or soon after giving birth. This does not mean that you should avoid traveling while pregnant, but it does mean that you should reduce the risk by keeping your legs as active as possible.