I truly have no idea how I would react until I was actually in that situation.
I got pregnant at the age of 19 and had my son about a month before I turned 20.
My parents didn't go off on me or anything like that but they did tell me they were disappointed. I think they handled it better than most parents I hear about.
Plus they had already been through the situation before my brother drew. He and his gf (ex now) had a kid when I was 16 years old, she had just turned 18 and my brother had just turned 19. The only difference with that one though is my parents found out from her parents when she was 7-8 months along so it didn't give them enough time to prepare. I told my parents I was right away, probably a week or two after I found out (and I found out at 6 weeks).
I would hope that I would have talked to my son enough about sex and the consequences with it that he would make choices that wouldn't put him in that position that early in life. I also hope that he would feel that he could talk to me about anything and come to me is something like should happen while he was a teen.
Yeah I know I would be upset and dissappointed but I would be there for him and guide him as best as I could.