Pregnant???

Well im 18yrs old i live with my boyfriend we have been datin for almost 3yrs now...well i think i might be pregnant i told him and he was all excited cause he wants kids really bad and so do i this isnt the problem here's the problem im a BIG daddys girl always have been..and i just dont know how to tell him that i might be pregnant should i wait intill i know for sure or should i go ahead and tell him now?? im the youngest out of 4 others so im the baby and i just dont want him to get mad at me or sit down and have one of them talks....i know there is nothin he can do about it but i want him to be there for me and support me....so how do i go about tellin him???Plz help..