A pregnant woman should generally stop flying at around 36 weeks of pregnancy. If she is carrying multiples or has had pregnancy complications, it is recommended to consult a physician and possibly stop flying earlier, around 32 to 34 weeks depending on the situation. Most airlines enforce a cutoff at 36 weeks for single pregnancies and may require a medical certificate after 28 weeks. Flying in the second trimester (3-6 months) is considered the safest period to travel. After 36 weeks, the risk of labor and complications increases, and many airlines will not allow pregnant women to fly. It is also advised to get medical clearance before flying and inform airline staff about the pregnancy for better assistance.