Telling my best friend I'm pregnant?

I've been best friends with my best friend since we were very little, we grew up together. My freshman year of high school I was diagnosed with a sever A typical case of PCOS (poly cystic ovarian syndrome) and was put on meds. Four years later I was told there was little to no chance I would ever be able to conceive and have children. Needless to say it was devastating. My family as well as my best friend and her father were there for me as well. My best friend went off to college the fall after I received the news and me and her father started hanging out. Out friendship blossomed into a relationship, and this last summer we "came out" to his daughter. My best friend has been accepting of this and has dealt with it as best she can. I totally understand that it was not easy for her to accept and come to terms with. I feel lucky and blessed to be able to keep both my best friend and the man I love. However I think that my best friend will have a hard time with the greatest news I've ever heard. With my PCOS infrequent menstruation is normal, but after not feeling well i went to my doctor and after doing some tests it was confirmed that I was pregnant, 2 months along. Now my question is how do I tell my best friend I'm pregnant with her father's child and her half sibling without putting her through the same turmoil she went through when we announced we were together?