Damn, just typed a long reply and lost it!
I agree with Ruth, but I read it as being a three month probationary period, with a chance that Castanet would be told at the end of the probationary period that she wasn't up to scratch and therefore wasn't being kept on. The same thing would apply if it was a fixed term (although the legal analysis of the situation is a bit different- it is a failure to employ rather than a dismissal - the practical tips are the same).
If I were you, I would try and force the manager to have a conversation about how your trial period is going. Be enthusiastic and positive. Then, after he has said positive things about keeping you on (which I assume he will) tell him your good news. As a practical point, I would tell him that you didn't know when you took the job (you don't legally have to, but I would) and you might also want to tell a little porky that it was a surprise even if it wasn't (bosses sometimes feel annoyed when women join and then go straight off on maternity leave. The 'ooh it was an accident' line can help it smooth over at a purely emotional level!).
I would not tell him your news until AFTER he has said positive things about your performance during the trial period. That will make it more difficult for him to backtrack (if he's thinking of that. Sorry, I'm a dreadful cynic).
Some employers will, I am sorry to say, still suddenly decide that you have not passed your probationary period/ your position is not being made permanent. You then need to decide whether you take legal action. Making sure that he has said positive things about keeping you on before he knows about your pregnancy supports you if you do need and want to go down that road.
Good luck. I hope that he's one of the many good bosses out there who (whilst possibly a little peeved that he's losing a good new person so soon, which I can sympathise with) is genuinely happy for you and glad to have you back after your maternity leave.