As a recruitment consultant, I'd say there is no easy answer to this - you are the best placed to make the call.

It depends on a number of factors including your length of service with the company and what policies they have in place for maternity cover and the like.

At the end of the day, they're going to find out sooner or later - the best thing you can do is make sure that they find out on your terms and that you retain control of the situation.

Good luck (and congratulations)!