Tell them naturally,like you would tell another friend. I went through the same. These friends dont want to be pities, and they definitely dont want to find out from the street about your news.
Please Please Please- dont give them the Bracha- IYH by you! It hurts them a lot. You can think it, and Daven to Hashem, but dont show them that they’re world’s pity.
And during pregnancy/ child ubringing, dont act as if you dont have kids. They will know you are trying to avoid these conversations. Of course, dont tlak aobut it a lot, or complain ever, but mention it here and there. This makes them feel normal. Their biggest worry is that they are world’s pity.