Why would my parents disown me if I got pregnant at 18?

Today I told my mom that me and my boyfriend are trying to conceive. My mother said, if I got pregnant now, she and my entire family would disown me. Why would they disown me? It's really not a big deal if I have a baby right now. I think that I am ready to be a mother because I am almost in my 20's and I want to be pregnant already as soon as possible. My aunt was not married and she was young when she had her first baby. How can I convince her that it's not a big deal to be pregnant at my age?
Why would my parents disown me if I got pregnant at 18?
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