Technically, you can fly anywhere up to 36 weeks. Once you hit 36 weeks, most airlines will refuse to let you fly. It is possible to get around this rule with a doctor’s certificate saying they are okay with you going aloft. In fact, if you look like you are nearly the end of your pregnancy; such a certificate might be required so the airlines have proof of how far along you are.
If you are having more than one baby, the recommendation is not fly comes sooner due to the higher risks associate with multiples including a greater chance of preterm labour.
Technically, if you are in your first or second trimester, there is nothing to keep you from getting on an airplane. The nausea that comes along with the first three months of pregnancy may make flying uncomfortable, but that does not mean it is not safe.