Talking with my mom?

I am pretty close with my mom and we are pretty open with each other. I started having sex with my boy friend a couple months ago and I kinda want to let my mom know because I trust her and I don’t like keeping secrets from her. I also feel safe with her knowing incase I ever have a pregnancy scare or an issue. I feel like she’d take it ok if I told her because one time my bff had a pregnancy scare and me and my mom bought her plan b and my mom told me that she hoped if I ever had an issue like that, that’s I’d come to her and be honest. But that was my friend not me... Should I have a conversation with my mom or just leave it? Also, if I should talk to my mom how should I start the conversation? What should I say?