Milah and Length of Purification Period

"The day when the father performs milah, the first duty a man must perform for his son, should imbue the father with the sacred resolve to "raise his son to walk in moral strength before God, the God of the Law," and to serve his son, by his own conduct, as a model for such a way of life. Similarly, the fact that the woman's path to purity following the birth of a daughter is twice as long as after the birth of a son certainly should serve to impress the mother with all the solemnity and grandeur of her own task to become teacher, guide and model for the Jewish woman of the future. After all, the mothers' influence in the molding the moral standards of her daughter is twice as great as her influence on the moral development to her son. With sons, the main thrust of their training comes from the father, to whom they can look as a model for the own future male role. With daughters, on the other hand, the mother is not only their role model but also the molder of their character, so that after the birth of each daughter she will do well to prepare herself with redoubled intensity, both for her own sake and for the sake of her newborn daughter, in order that the both may ascend the lofty path or purity and morality to the heights of God's own idea of holiness." R' Samson Raphael Hirsch, Vaykira 12: 4-5, The Chumash, Judaica Press, Gertrude Hirschler translator.