Paula England reported at the Furstenberg Conference on her new study of class differences in having children. She found that girls from all classes want the same number of children - on average, a little over two. However, by 16 there is already a negative correlation between sexual activity and income/GPA. That is, the poorer girls, who are also likely to be the girls doing worse in school, have sex more often than the richer girls, who are also doing better in school and are on the college track.
Eventually, the dropout girls have four times the unintended pregnancies that the college-track girls do. It is not that unintended pregnancies derail some girls from the college track - the causation runs the other way. Girls who start out poorer are likely to be "sloppy and inconsistent" in using birth control, whereas the middle class, college-track girls are not. Moreover, England reported, this class gradient in birth control goes back at least to the 1920s.