Telling parents?

My SO and I are both 20, and this pregnancy was completely unplanned. Its been about 3 1/2 weeks since we found out, and now that the shock is over and we've decided to keep the baby, just one more problem is left to face: our parents. I'm terrified of a bad reactiom from both sets of parents. I don't even know how to begin the conversation... has anyone else been through this? Advice would be wonderful. I want to tell them, but I don't know how. But I can't hide it much longer.