how do I ask her?

My boyfriend and I have been dating for 9months and we've been having sex the past 4months already. I haven't told my mom yet and every time she asks if I'm still a virgin I lie and say yes. I have a feeling that she might know the answer but I feel bad every time. How do I let her know I'm ready to have sex with my boyfriend? (I still don't want to tell her that I already have.)