Evan of Two Puppies Enter, One Puppy Leaves posted a link to a BBC science podcast in which scientists explain that three years after the birth of a child, oxytocin receptors in the parents' brains wane, while vasopressin becomes more prevalent.

I don't have time to listen to the podcast or follow up on the journal articles right now, but according to his blog post, Lucy Vincent (neurobiologist at the French Scientific Research Center) and Dave Perrett (Professor of Psychology at the University of St. Andrews) said that

after 12-18 months the infant becomes significantly less vulnerable (able to stand and perhaps throw off a bird or small animal attacking it), at which point only one parent would be necessary. And at that point, the oxytocin effect more or less ceases, yielding to increased vasopressin receptor activity- essentially a biochemical foundation for why people tend to “fall out of love” after a few years

I think maybe they were talking about the male brain, not the female's.

Motherhood and fatherhood both change a person's brain; changes in the mother's brain have been shown to be permanent in rats. However, oxytocin receptors, which become profuse during pregnancy, may disappear and/or become less sensitive.

Vasopressin, which is very closely related to oxytocin, may be more involved in male attachment, and lead to the expression of this attachment in protective behaviors. If you have the time, his post and the podcast are probably worth checking out.