They usually allow up to 32 weeks with fit to travel certificate. I think they will not let you fly once you have reached 36 weeks as you may go on labor anytime (even if you have a scheduled cs). You have to inform your OB of your plans. Contact the airline about their restrictions for pregnant passengers. The airline will have you sign a waiver that they will not be held liable for anything and if anything happens to you while on flight, you will reimburse all expenses to them.