Why is it unsafe to fly in pregnancy?

Not necessarily: Whether or not you should fly in pregnancy depends on how far along you are and where you are going. Long plane rides can increase your risk for a blood clot so you should drink plenty of water and walk in the aisle every hour or so. You don't want to risk having the baby away from home so don't travel far during the last month of pregnancy.

Labor.: It is not really unsafe in and of itself. However, it is not pretty if one goes into labor during the flight, especially if the baby is premature.