I need ideas on how to tell my mom we’re having another baby

We have three older kids (each of us has one from a prior and we have one together and we’re baking our second). My mom is kind of a nay-sayer who’s a feminist and kind of hates that I’m a stay at home mom and not hugely into a career. She has repeatedly made comments about me getting fixed or not having any more babies. I’ve told her with both other pregnancies right away, but didn’t want to hear her bitch this time so I only told my grandma (they live together). Anyway, her birthday is in 2 weeks and we’re going to have the kids tell her that day, but we aren’t sure how to go about it. Anyone have any cute ideas you could float my way??