Oh wow. Thanks so much for the detailed reply, that's both fascinating and frightening.
I was once advised not to tell a potential employer I was pregnant during the interview, for a strange reason: because (it was argued) it would put the potential employer in an awkward legal position if they offered the job to someone else (ie I could potentially argue I had been discriminated against), and therefore I shouldn't tell them, for their sake.
But clearly that's not the case at all, and it was an overly optimistic view of the way things work in practice...
(Not that it mattered anyway - I did not tell them, but for my sake!)