You need to have a transvaginal ultrasound. I'm actually surprised that they didn't do one-if you are pregnant, you are way way too early for a pelvic ultrasound to catch anything at all. Call them and tell them you want a transvaginal or another blood test.
Also, I do not intend to judge you, but I do not believe you are being realistic here. I don't believe that your fiance can support your child on one income (btw, how old is he?) and it's obvious that at your age you do not have a career of any kind or education yet. Odds are, at sixteen this isn't the first relationship you're going to have. I just feel that getting married and being a married couple will do you a world of good and mature you a lot more than you think. You have no idea how important that time together is until you throw it away.
I just think that if you WERE grown up enough to be a mother, you'd give your child more.