Traveling by air is generally considered safe for women while they are pregnant. Many physicians recommend that a woman not fly during the final trimester for many reasons, but often as a precaution in case of an early labor. The second trimester is generally considered the safest time for flying, having made it through the critical early months and not being in danger of an early labor.
You should consult the airline rules before planning to fly while you are pregnant. Most airlines allow pregnant women to travel up through their eighth month. Traveling during the ninth month is usually allowed if there is permission from the doctor. An airline generally depends on the honesty of the passenger as to how far along she is.
You should be aware that most airlines have narrow aisles and smaller bathrooms, which makes it more challenging to walk and more uncomfortable when using the restroom. Because of potential turbulence which could shake the plane, make sure you are holding on to the seat backs while you are navigating the aisle. You may want to choose an aisle seat which will allow you to get up easier to reach the restroom or just to stretch your legs and back.
Keep your air travels on major airlines with pressurized cabins and avoid smaller private planes. If you must ride in smaller planes, avoid altitudes above 7,000 feet.