I would personally wait until the first trimester has passed. That golden window of opportunity nature has for deciding for us whether or not the fetus will in fact come to fruition. Sorry, don't mean to sound negative, or gloomy and I DO congratulate you, my dear heart, on being pregnant! Babies are a touchstone of God that I happen to adore with all my heart!
I just feel (a bit of a superstition) that anyone shouldn't tell anyone else at work or otherwise, about a pending/new pregnancy until that first trimester is officially passed on the ol' calendar.
Now, after that first three months, I say go ahead and let NM know so that he/she can be ruminating over several things regarding the upcoming months of scheduling. As you are well aware, that will be her/his biggest concern (aside from when and if you are coming back after baby is born)
and must be dealt with in some fashion. As well as contacting HR, which would probably best be handled with you informing them as well as your NM at or about the same time after first trimester.
Good luck and take extra good care of yourself. Have a wonderfully joyous