At what week must you have a docs letter to fly? And how dangerous is it to fly after 28 weeks

Most airlines require a letter after 28 weeks of pregnancy while after 36 weeks pregnant, many airlines will not let you fly due to the increased risk of delivering on board. The main risks of traveling during pregnancy are increased risk of blood clots and Deep Vein Thrombosis. https://en.wikipedia.org/wiki/Deep_vein_thrombosis