Since I'm neither a teenage girl in high school (I'm a male, in college), nor have I ever been pregnant (again, I'm a male; the closest thing I'll ever have to experiencing an actual pregnancy is playing The Sims 2 without cheat codes), I cannot describe exactly what goes through the mind of a pregnant teenager who decides to go through with giving birth.
However, the reason this thread exists is because most people believe that pregnant teenagers came to be that way through bad decision making, and that being a teenage parent is inherently a big mistake. That's a load of crap.
First of all, it's not like there's anything inherently wrong with being a teenage parent; it's not like any child born to a mother under the age of 20 is going to be at any disadvantage (or be born with gills).
Secondly, while it's true that most teenage pregnancies are unintended, they are almost never caused by a lack of education, maternal instincts, financial problems, mental disorders or sheer ignorance; these are just the causes and explanations we're exposed to the most often. The result: everyone who's ever taken a SexEd class now has a Confirmation bias, and thinks that teen pregnancy occurs solely those reasons.
And finally, sometimes, believe it or not, it's the right decision. I'm good friends with a girl who had a baby during her last year in high school; and as of right now, her life is probably a whole lot better than mine. If being a teenage mother was the hindrance most people believe it to be, that probably wouldn't be the case now, would it?