Kelly Musick and colleagues report in the current Social Forces that both groups of women want the same number of children. However, educated women are better at sticking to their plans. The less education a woman has, the more likely she is to have unintended pregnancies.
The sociologists distinguish among intended, mistimed, and unwanted pregnancies. Mistimed means "I wanted to get pregnant in the future, but not when I did get pregnant"; unwanted means "I did not want to get pregnant at all, but I did." Both of the latter are "unintended."
The core finding is this:
What education mainly deters is unintended births. ... The least educated white women are predicted to have .86 times as many intended, 3.02 times as many mistimed, and 6.68 times as many unwanted births as their counterparts who have graduated from college. [The comparable numbers for black women are 1.36, 1.69, and 7.33]