Telling the boyfriend?

How the hell do I tell my boyfriend that I'm pregnant? We weren't particularly trying to conceive. I thought I might've been pregnant a couple of months ago and when it turned out I wasn't he was relieved. He's going to think I planned this and be upset with me and that's the last thing I want. We've only been together for 6 months, and we've talked about marriage and kids in the not too distant future. I'm scared he's gonna be mad, and not really want the baby. But I'm over the moon completely.