Need Help

I am a young mother to be. I am 20 in 3days. Bieng that i was so young my mother never supported me. The boy I ended up pregnant with has proposed we have been together since 4th grade and we have a apartment together. But I often find that my mother shows up and is constantly nagging at me which is causing a strain on our relationship and stress between the baby and I.  I do not want to cut my mother off but I can not find a way to tell her she is causing a strain its actually getting so bad my Fiance cant even engage in sexual things because we have fear of my mother dropping in. My Fiance has told me if i do not tell her to stop our relationship will fail. My mother often says. that we are too young and has even says she wants to take our child.  Any advice I do not want to loss my mother nor my relation ship but I am so stressed its causing alot with the baby.  Am i wrong? Should I let my mother take the child because I am too you?