Openly and honestly.

It depends, is this a reason for rejoicing or is it a reason for sorrow? If it is a reason for rejoicing, then you could just tell him straight out or you could play some little game with it. This reminds me of an episode of "I Love Lucy", a 1950's situation comedy. In it Lucy learns that she is pregnant, but keeps getting interrupted every time she tries to tell Ricky. So, she eventually goes down to the night club and has an anonymous note passed to him requesting a special special song to help get the courage to tell him. Ricky, not knowing that the note is from her, decides to start singing "Rockaby Baby" as he goes through the audience trying to find out which couple it is. When he gets to Lucy, he just says hi to her and doesn't think anything about it until she nods at him. He then does a double take, she nods again, and he gets a stunned look on his face. Then the celebration begins. So, if this is a good thing, then be creative. Have some fun with it and make the moment special. On the other hand, if this is not a joyous occasion, then simply sitting down and telling him straight out. You two are going to have some decisions to make and it would probably be better to keep this serious so that you can keep clear heads as you discuss and make those decisions.

If you're happy about it, do something fun! When I got pregnant with my son, we'd been trying for awhile. I bought a baby rattle and put it in a gift box, and put it at my (now-ex)husband's place at the dinner table, and had him open it. If you're not thrilled with the situation, tell him that you need to have a serious talk, and then do it, as soon as possible.