How do I tell my mom

I just found out I'm pregnant and I am scared to tell my mom because I know she's doing to be disappointed in me for not being married. I'm 20 and my SO is 22 we have been engaged for over a year just haven't gotten around to planning wedding because we've been so busy. We both have good jobs I run my own business I just want her to be excited but I'm worried she won't be.