First, I would be happy to do so if she looked at all pregnant. Now, she may be (skinny jeans aside), but I'm thinking, not so much. Many times while riding the train pregnant, people offered up their seats to me, and when I was huge or very tired, I gladly accepted.
Second, use your big girl words if you feel that I don't understand your needs. "Excuse me, could I please have your seat. I am great with child and need the rest." To pat your tummy and not communicate to me is to be petulant.