Obviously I am aware of the fact that it might take us a while anyway, that you have the legal right to get pregnant whenever you want, and the risk of them getting rid of you for some spurious reason before you've been there for 2 years. I've also looked at the number of weeks for which you'd need to be employed before you were entitled to SMP etc.
Practicalities aside, though, I feel very guilty - I suspect it's this kind of thing that disadvantages women in the workplace generally. I'd be keen to stay with this firm for a very long time so I guess it all balances out over the duration of a career, but it is still likely to be a bit awkward. In all honesty, what's the minimum amount of time you think is decent in a new job before you go on maternity leave? A year? Two? More?