Help! How do I tell my parents?

I am 18 years old, my fiancé and I have been engaged for 3 months. We both have stable jobs and a house but right now I'm staying with my parents because my fiancé is making a few renovations to the house so it will be more child friendly and to fix the flooding we had in the guest bath for which we're having to replace the floor from the bathroom to halfway down the hallway. My parents don't know but have told me before that they rather die than me have a child with my fiancé (even though it is a miracle that I am pregnant due to the fact that the doctor said I had a slim nearly nonexistent chance of becoming pregnant and a accident baby beings it was our first time and the condom broke) how do I tell them without them disowning me???