On a bus who has right of seating?

If you need the seat, you're entitled to it. Just say, my doctor advises me to sit on the bus. Don't argue. You don't need the grief.

I don't think that officially any of the categories of people mentioned on the sign have priority over the other people mentioned on the sign. I know what you're experiencing, because when I was 8-9 months pregnant, I rode a bus to work, and nobody ever offered me a seat.

If you feel you would be in real danger by standing, you should say to people who are yelling at you that it's not safe for you to stand because you are pregnant and have balance problems. Make a general request that someone else give up their seat for anyone else who needs to sit down. That should shut them up.

You completly deserve a seat! Being pregnant disables you a bit, with the huge stomach and you totally should sit. Maybe next time just say politely that you are pregnant (as if it weren't obvious) and you need to sit, if they continue to yell at you then just ignore them

priority seating is exactly what it is, you are pregnant and deserve to sit. not only do you deserve to sit but you should be offered a seat and not yelled at.

other people who are standing up have no right to tell you to get up. i also believe you should tell them you are pregnant and it is dangerous for you to stand. you should politely request someone else to get up.

hope it works out :)

Spray mace at whoever complains to you.

Just kidding. The other answers were very good so I had to say something silly :)