Some parents just simply aren't responsible enough. However, if abortion ever is outlawed, it would start a vicious cycle of terrible lives and childhoods.
Imagine this, a girl gets pregnant at 16 and is forbidden to abort it. She basically ruins her whole future trying to juggle responsibilities and a child that she has no knowledge about. Soon, the mother is broke and the father walks out while the family is disowning her for making an irreversible dumb mistake. The child now has little food, little education, and poor living conditions. This will affect the child all of its life, as it will never be able to find a job or be successful. This will lead the child into making poor life decisions, such as drugs, alcohol, ad sex. Assuming the apple doesn't fall too far from the tree, the child will make the same mistakes its mother made, thus continuing the cycle of poorly raised children that cannot raise their own children and will not learn from their mistakes.
I don't know about you, but I think it's better to sacrifice one life than have 10 generations of failures. Pro-choice, abortion should be legal.