What is wrong with offering a seat? Is there a halacha against it?
Firstly, you may never know if she is pregnant. Secondly, it can be a huge kiddush Hashem… you never know how many “looks” I see being given out on the subway when a frum yid sits and someone else stands. If you are strong and capable of standing on the subway/bus and someone else looks like they could use the seat, no matter the age or gender- offer it! I have seen young men and women who look like they would collapse from their bags, mothers with small children, men who look like they will pass out from exhaustion… and I offer a seat to them.
You never know what physical ailment someone may have and they may not want to open up to a stranger. Just offer if you can.