Before revision: According to one theory, It evolved in some placental mammals because it confers advantages in that it allows females to detect the genetic abnormalities in the embryo and reject it, thus allowing the mother to limit her investment in bad embryos.
After revision: It evolved in some placental mammals because it confers advantages in that it allows females to detect the genetic abnormalities in the embryo and reject it, thus allowing the mother to limit her investment in bad embryos.