From the first day you know you are expecting a baby, there are some key points to think about concerning safe travelling in pregnancy. However, the issue of your seat belt - in a car, coach or plane - probably doesn't become a worry until your bump starts to show.
Always wear your seat belt
Even at full-term, your bump is better protected under a seat belt than it is without one.
In the early weeks of pregnancy, you shouldn't worry about the pressure of a seat belt at all. If you feel unwell or have stomach cramps, these will not be caused by a safety harness. Your womb is well protected in there!
As your bump grows, try to keep the lap part of your belt low, under your bump if possible. This is more for comfort than safety, as it should not pose a problem for your baby, even if worn normally.
If you are involved in any road traffic accident, tell the attending services or the casualty department that you are pregnant. They can then do checks to put your mind at rest or to give you any neccessary treatment.(In the event of a serious accident, medical services are amazing at care for pregnant women and an unborn baby. Dwelling on worst case scenarios when they probably won't happen does not help you in a regular pregnancy.)
If you have just had to stop short in an emergency stop, don't worry that the belt will have tightened across you. It might make you feel more sore, but the fluid in your womb is designed to be the most amazing 'air bag' system in the world!