Leviticus 12:2

From uncleanness contracted by the touching or eating of external things, he now comes to that uncleanness which ariseth from ourselves. She shall be unclean; not for any filthiness which was either in the conception or in bringing forth, but to signify the universal and deep pollution of man's nature even from the birth, and from the conception. For seven days, or thereabouts, nature is employed in the purgation of most women. For her infirmity, i.e. for her monthly infirmity. And it may note an agreement therewith not only in the time, Leviticus 15:19, but in the degree of uncleanness, which was such that she defiled every thing she touched, &c.