A woman who is 34 weeks and under in her pregnancy can fly without restriction. However, the first trimester (1-13 weeks) may be difficult for some women since there is a lot of morning sickness and this period is when they are more susceptible to miscarriage. Additionally, some airlines require that a pregnant woman sign a "Release & Indemnity Form" to free the airline should anything happen to her on the flight. In general, the second trimester (14-28 weeks) is a good time to fly, since most women feel more comfortable and stable, while the risks of miscarriage and premature labor are lower.
Women 35-37 weeks pregnant are generally not permitted to fly due to the risk of premature labor. In some situations, a "Fitness to Fly" certificate and a note from a doctor, stating that the pregnant woman is in good health and does not have medical complications, may allow her to fly. The certificate must be signed and dated one week prior to a flight, and a pregnant woman can be rejected by the airline without such a notice.
Women who are more than 38 weeks are not permitted to fly under any circumstances. This is to prevent preterm delivery as well as for the safety of other passengers.
International flights (or flights over major bodies of water) generally adhere to the domestic standards for pregnant women. However, all international flights require that a woman present a medical consent form from her doctor after the 29th week of pregnancy. With some airlines, a woman can request to fly within 10 days of her due date with a medical form, but in general she will not be permitted to fly after her 35th week of pregnancy.
