no idea how to tell her I'm pregnant

I/fiancé and I have no idea when or what to tell his mom. We live with his mom and step dad. They love having us here and we pay rent and such. Financially we would be okay we plan to move just so we have space. I am 22 fiancé is 20. I just have no idea how to tell her. Then they are always talking abou future trips and big birthday ideas and I'm ruining it all I feel like cause I'm having a baby. They have nice things and everything a certain way. We aren't savers and obviously have to change that. I'm just deathly scared to tell her. Scared she'll be so disappointed scared she'll hate me. Scared I ruined her sons life. It's like they want us to live life they way they do and we don't. Fiancé is taking over his stepdads business and will make more money having it then if he went to school for a degree. I'm scared they'll be like you should've done this and that. Any advice. I can't even enjoy this pregnancy even in the slightest because I'm so scared.