Hi Jillamona,
my theory is that people 'mirror' your emotions about pregnancy.
So if you say "I've got some fantastic news... I'm pregnant! Woohoo!" they'll be happy for you, because you are happy.
But if you say "Oh. My. God. I can't believe I'm pregnant." people will worry and say "oh shit, are you okay?"
So if you are happy about the news, let them know it. After all, most parents just want to know that their kids are happy.
But I also agree with Boo81 - they'll definitely worry, that's their job.
It might be worth letting them know that you HAVE thought about money, college, time off, housing, your relationship etc. It is okay to admit that it wasn't planned. Give them time.
But I reckon all parents come 'round pretty quickly, once the realise there's a lovely, cuddly grandchild at the end of it.
Good luck and congrats :)