Teachers who have had children, when did you notify admin of your pregnancy?

I'm sure this has come up a few times so I apologize for redundancy.  

I'm in my mid-twenties and a second-year teacher at a religious high school. My husband and I just learned that we are facing a completely unexpected (but not necessarily unwanted) pregnancy. I'm only 4 weeks right now (due on Halloween!), so it's going to be a while before I tell ANYONE, but I know that it's going to be very hard to hide in a few months.  

Given our church's official stance on "the family" (take that as you will), my admin has historically been very supportive of new parents. I have also received very positive evaluations and a number of positive comments from admin about how much I've grown as a teacher this year. All that aside, I still feel a bit apprehensive as our contracts are year to year and aren't given out until mid-May.  

Those of you who have had remotely similar experiences, what did/would you do in terms of informing admin/department chairs/etc?  

tl;dr: I'm growing a tiny potential person. When should I spill the beans to admin?