Teen pregnancy

When should I tell my mom I'm pregnant ? I am 18 but I am def still my moms baby and she will not be happy about it . My mom knows I was suppose to start my period . It's been almost a week and she hasn't brought it up . But How should I tell her ??? Any good words to use ??? Should I wait till I get my ultrasound and everything is okay ?! Or just tell her now ? I'm about to be 5 weeks . Should I tell her the day I turn 5 weeks or wait ?? I'm soooo scared . Me and my boyfriend have a lot under control so that won't be a problem but disappointing her is not easy for me