I wouldn't be telling them right away... wait until atleast 12wks, you are so early on and anything can happen (not that it will, but these things do happen). Even when you do tell them, you will still be in the role even though you are on maternity leave at the time, it shouldn't make any difference.
Good luck with it, you are very lucky to be getting paid m/leave too. Wish we were all in the same boat