If you intend to continue working for them and return after maternity leave then I wouldnt tell them until its obvious in the pregnancy you would want to have to tell them all your personal business unless necessary.

I didnt

And find out whether you are even entitled to maternity leave before a full 12 months work, because I wasnt at 11 months So had to resign and get no money or job back

Oh well I like not working better Best of Luck !!!!