scared to tell my parents I'm pregnant

I'm 21 with my own house and have been with my fiancé for 3 years.. not as if I'm 16 and pregnant from a one night stand 
I'm so scared to tell them. As they still treat me like a child . I only look about 15 and I'm 5ft. 
How should I tell them? I don't feel comftable enough to tell them face to face
I don't know what's wrong with me!!!!