What causes teen girls to ignore a pregnancy until the birth?

I’m talking about the occasional case of a young woman who somehow ignores a pregnancy until a baby suddenly arrives. This often ends in tragedy when the girl panics and abandons the child. What influences could lead to this behavior?

1) On a small scale – what sort of family dynamics might she have, what sort of personality, etc. What might pre-dispose a young woman to ignoring a pregnancy?

2) On a larger scale, is our society somehow leading to this problem? Is there such shame involved with teen pregnancy? Is it a lack of resources for mothers? Is it an emphasis on career over motherhood? Or, could it be an emphasis on avoiding abortion? Any and all ideas are welcome.
A pregnancy is often not seen as a blessing, even for women with adequate resources. Even a surprise pregnancy in a 25 year old with a job is greeted with “oh no!” by friends and family. That’s what I mean by “an emphasis on career over motherhood”.