Just be straight forward and say look I don't know how to say this so I'm just going to spit it out I'm pregnant.
You could always say I told you you should have worn a condom.
but there's no point in pointing fingers it's an issue that you need to deal with and it'll give you a good idea of what kind of a boyfriend you chose. How he deals with this problem or issue I shouldn't call it a problem will tell you a whole lot about how he is as a person.
This is actually something that you have to deal with on your own if he chooses do you have an attitude about it.
Feel free to p.m. me if you want. There's really no advice I can give you but I will pray that you have a decent boyfriend and that you and he can come to an amicable agreement. No matter what you choose.