I am 23 now, but I was 18 when I got pg with my husband. My parents still got very upset and we didn't talk for a month. Even though I was married, had been with my husband for 3 years already and we were doing well on our own.
Honestly, I think parents do this because they have a hard time with accepting that you are growing up. Don't worry, if at first they get upset - they will get used to the idea quickly most likely.