How should I tell my grandma I'm pregnant?

So I'm 21 and my grandma raised me. I'm almost 8 weeks pregnant. I want to tell her I'm pregnant but she's also Roman Catholic and Catholics are severely against getting pregnant outside of marriage. My boyfriend and I already planned to get married it just so happened that I got pregnant first. I want to tell my grandma because they are estimating my due date at September 25th and her birthday is September 26th. Plus I want to tell her because her other great grandchild she never sees because my brother doesn't want anything to do with his dads side of the family except our dad. Any advice on how to tell her? And what should I say or do if she starts lecturing me on how I'm sinning because I'm not married yet and I'm pregnant?