I would accept now and tell them when you are good and ready and not before.

They are practical people (I also work at a university). They know that

- you might get a better job at a different university and leave

- you might get pregnant next month, or the month after, or whenever, and it will still have implications for your career. These things happen to women and they do know about it.

- you might leave your job altogether for any reason, with little notice.

So although they plan things 12 months in advance, I think you have no reason to need to tell them about your pregnancy until months down the track. And I wouldn't do so if I were you. Universities, though huge and lumbering beasts, are also good at "making do" when people's position changes for whatever the reason.