Ooohh. Then you open with "Mum, dad, I'm dating a muggle and I got her knocked up, we're going to get married and move to America."

Then when they're done having a heart attack laugh and tell them you were just joking. Just about any news aside from "I'm a murderer." can follow that and it'll be a bit more in perspective.

Depending on the parent they might take "I'm a murderer" better after hearing that, too.