well i dont know what airline were going with yet but i found this and it sounds pretty reasonable...
Expectant mothers travelling on a Thomas Cook Airlines flight can be accepted for travel up to 28 weeks without a medical certificate. When travelling between 29 - 34 weeks a medical certificate confirming the number of weeks of pregnancy is required confirming that you are fit to fly. Pregnant passengers cannot be accepted for travel after week 34, and so all return flights have to be completed no later than week 34. All medical certification must be supplied by a doctor - medical certificates completed by a registered midwife cannot be accepted.
Ive flew 6 times all together while being pregnant, all to spain, and the last time i flew i was only about 21 weeks pregnant and it wasn't very visable so i diddn't get asked questions or anything, but i don't see that my doctor will have a problem writing me out a certificate.