Plane travel is safe in pregnancy despite some voiced concerns regarding cosmic radiation and low oxygen levels in the passenger compartment. The risk in both cases is negligible.
X-ray machines are felt to be safe to go through while pregnant. Try to get an aisle seat so that you can hydrate and be able to go to the bathroom frequently and take repeated walks down the aisles to decrease chances of a blood clot to form.
Attach your seat belt below your belly. If you are in your third trimester(weeks 27-40) and the flight is over a few hours, you may experience significant foot swelling, so consider using comfortable sandals and support stockings.
Finally, make sure you are aware of the airline’s pregnancy age cut off. Many use 36 weeks as a cut off, but some use an earlier age. It is always a good idea to get a note from your health care provider regarding your due date and exactly how many weeks pregnant you are, since the airline might ask for it.
If you have any contractions or bleeding, do contact your health care provider before leaving as they may not want you to travel. Finally, if you are prone to nausea, ask your health care provider for some nausea medication that is safe to use.