Hey girl! Your thread caught my eye. I was once in your position and I can tell you in simple terms.. no, it's not a *shared* decision. It's 100% your decision to make.
I completely understand your fears over loosing your boyfriend, but please understand that no matter what you do, or what you don't do, it wil be his choice whether to stay with you or not. I mean to say that there is nothing in all of reality that you can do to keep him or make him leave.
My boyfriend (now husband) had his mother & sisters in his ear. I had jus turned 17 and he about to turn 18 when I got pregnant. She convinced him that we could not handle having a baby, that I was trying to *trap* him

, and that we wouldn't ever make anything of ourselves if I kept the baby. As a matter of fact, she was going to take me to the clinic to have the abortion. But thank God I changed my mind. He'd threatend me that he would leave if I kept her and all that stuff and I was terrified. But I came to realize that there is no guarantee that he would be a great guy to have around, or that I would be better off without him. So I had to make the decision for myself.
We do have a happy ending. We did *make it*. ANd you know what, his mother even apologized to me and thanked me after all that I kept the baby. My firstborn is now almost 18. I look at her and think "Wow." just wow. It did take him a while to come around, but.. he did. We've been married now for over 10 years and have added 4 more kids to our brood.
So chin up. It will be ok.