How to tell my fiancé!?

So, here is the deal. My fiancé and I already have 2 children. My daughter is 10 and my son is 3. We had absolutely no intentions of having a 3rd child. But, I think I'm pregnant. Unfortunately it is not exactly a good thing. I mean, it's a good thing, of course, but it is very unplanned and it is going to change everything from our plans for the future, to our house constructions plans, to when I will actually go back to work. I do not know to tell my fiancé because I know he is going to be upset at first, and he will become instantly stressed out, being the provider of the household. Any advice mamas?