The rules are different for mothers who are high risk or are carrying multiples. The deadline for flight travel is even sooner than the third trimester. So, you will find it difficult to convince a doctor or airline to let you fly during the last month of your pregnancy but should be able to do so during the seventh and eighth months.
Anytime you are traveling while pregnant it is good to keep up and move around every hour or so to prevent blood clots. It is also important to drink plenty of water, avoid caffeine, wear your seat belt low and keep a list of emergency contacts on you in case anything does happen.