I'm sure this has been done to death, but I've commuted into London a couple of times in the last couple of weeks, and have never managed to get a seat. I'm only 21 weeks pg, but am quite slight so my bump is blindingly obvious (people ask to touch it!).
I know that it isn't always obvious when people need to sit down, so I really don't like asking, but at one point I was literally the only person in my carriage standing!
Am starting to think that all Londoners are mean-spirited and selfish (but then I suppose I'm being selfish in wanting to sit down). Or very unobservant.