Unless you are having some sort of physical issue caused by the pregnancy, it's just not necessary.
When I was 9 months pregnant with DS #1, I went to a local school on a Saturday, to sign up to join the JCC. As I was walking from my car to the entrance, I noticed another pregnant woman and her husband going the same way. I remember being fascinated and sort of horrified that when this couple got to the curb, they stopped, and the husband had to "assist" the woman in stepping up the curb onto the sidewalk. She couldn't lift her foot/leg.
It turned out that our babies were born on the same day. There was nothing "wrong" with her, except she let pregnancy become an excuse not to move.