Big city moms to be- subway etiquette

I'm living in nyc, am 15+2 and showing clearly already. Each day, I have a 90min commute (over an hour of which is on the subway). As I get bigger I'm getting more and more frustrated because I'm not getting seats on the train and standing is so painful on my feet/ankles, not to mention I'm in the phase where I've been getting mildly dizzy. Today, these two women asked for a seat for me, which was kind of embarrassing, although in the end I was totally thankful. What is the Etiquette when it comes to riding the subway? is it rude to ask for a seat? Should I just brave standing and count it as "excercise" (ha, I would totally count it). I know once I get bigger, people cannot pretend not to se me but in the meantime... Idk, I guess I'm one tired and grumpy mommy and am not sure what's the right thing to do.