You should tell them as soon as possible hun, peel the bandaid off! You'd be surprised at how understanding they could be. And if they are upset, you'd be surprised how quickly they get over it. I think every parent is worried about their child having their first baby but when the time comes, they'll more than likely be excited about the new little one! Good luck :)
Ive kinda gone through the same thing only with my grandparents. Im 19 and 11w 1d and telling them was the most nerve wracking thing ive ever done. I really thought that they would completely stop talking to me because they are very strict and old fashioned and believe in NO sex before marriage at all but anyways i told my grandmother a few days ago and she was surprisingly calm she wasnt happy in the slightest bit but no yelling or rude comments like i thought it would be. She pretty much told me to get my butt in gear grow up and get ready cuz it wont be easy. She said it is what it is and theres no going back now so we take it as it comes. She obviously gave me a lecture about money marriage and going to college because i just graduated high school but anyways she was way more supportive than i thought she would be. You just have to get over the fear and tell them. They may be a little upset at first but like my grandmother said, it is what it is and theres no going back now so tell them and expect that they will be a little upset but also remember that they will eventually warm up to the idea just reassure your family that you are aware of how big this responsibility is. Bottom line is they love you and care about you so it may take a little bit of time but family always has your back even if it doesny feel like it at times. Goodluck! Nd the sooner you tell them the better