I'm 19 (will be 20 in three months), and my boyfriend (same age) and I have been together for three years, and we're not only holding off on having a baby, we're also holding off on getting married, too. We're both in college and we ultimately want to go to grad school, too, and have successful careers and build our own life together. We plan to get married after we graduate from college, and we don't want kids until we're in our late 20s. It's important to have a couple years of marriage first without kids - so that when you do have kids, your marriage will be cemented enough to handle the pressures and stresses of raising a child.
I say you're too young because I know that i'm too young, and the reason I draw a comparison between myself and you is because you sound like you've got your head on straight, and that's definitely how i'd characterize myself. I never partied, I studied hard and made good grades and got into a major state university on scholarship, where i'm on the dean's list every semester and a member of honors. Every now and then I feel like I want to just get it all over with and be married and have a baby. It's those maternal instincts acting up! It's so important that you and your fiance are able to support a baby, and support him/her well. Think of how much more money you'll make if you already have a college degree and are starting a career. Your baby would be a lot better off if you had the money to care for him/her without the stress of not being able to make ends meet. And trust me, even if you go to the cheapest local community college, tuition bills on top of typical living expenses really adds up. They don't talk about being a 'poor starving college student' for nothing. I can't even begin to imagine how i'd make ends meet on a college student's budget with having a baby, and that includes having a job on top of financial aid loans. I also can't even imagine the stress you'd go through trying to study for exams, hold down a job, raise a child, and keep a marriage intact all at the same time. That's not saying it's not doable - yes, there are couples out there who are doing it and surviving. But my point is, why bring all that hardship upon yourself? Take life one step at a time... You're much more likely to stay in college if you have the time, money, and energy to focus on it. That's all the better for you, all the better for your marriage, and all the better for the baby you'll someday have.
There's one more reason, besides fiscal ones, that I think you should wait. Because you're so young, you're also very likely to change quite a bit. You probably won't have a stabilized sense of self until your mid-twenties. Until then, your opinions, perspectives, experiences, and general life views may fluctuate - and this could possibly tear apart your marriage. It's something that my boyfriend and I deal with all the time, and it's one of the reasons we're waiting to get married (also so we can totally focus on school and not the added stresses of a marriage, but that's just us, married people go through school together all the time and make it). Now, a divorce isn't easy (nor is it easy to even think about the possibility of it now!), but there's no reason to put a child through that too. It just makes sense, to me, to wait until you've been married a while, and have done all the things you ever wanted to do, first. Like going to college, and having a career. My boyfriend and I love each other and we have a great relationship and partnership, but we both know we're not ready for marriage, much less a kid. I don't know you, but rarely is anyone ever truly ready for that kind of thing at 18, though a lot of people think they are. Give yourself some time to enjoy being young. The time will come when you'll be all grown and ready to have a baby, so why rush it?
I hope this helps, and good luck with whatever you decide. :d