I didn't get offered a seat on the tube until my last day at work when I was eight and a half months pregnant. I only got one then because another woman who was standing said, very loudly, "do you find that people often don't offer you a seat?".
Have swapped London for rural France, but should I ever have to go back on the Northern Line again [shudder] I will try to remember to say that on behalf of any preggers ladies.
I asked for one of the tube badges (which say "I'm up the duff - get up will you?", or something like that) but the guy said that they were never delivered to the Northern Line stations. Fed my South London Northern line paranoia.