How do I tell my parents...

I'm 5weeks and 4 days. I just found out I'm pregnant and just got engaged to my fiancé. My dad was already a little reluctant to give us his blessing on marriage and I don't know how he is going to react to his 20 year old only daughter pregnant. He knows that I have PCOS and other reproductive issues and the doctors told me I'd never get pregnant naturally. How should I tell them?