Leviticus 12:1-2

2. She shall be unclean It is a mystery that marriage, a sacrament of love, prefiguring the oneness of Christ and the Church, should attain its divinely appointed end only by entailing ceremonial impurity. But nothing more impressively teaches the depravity of the human race than the early announcement that both conception (Leviticus 15:16-18) and birth are inevitably attended by pollutions which imperatively demand purgation before the person of the parent can be acceptable to the holy Jehovah. This suggests the strong assertion of David respecting the moral corruption of his nature while in embryo, Psalms 51:5. When Richard Watson was asked for the strongest proof text of inherited depravity, or original sin, he quoted John 3:6.

Seven days This number of days makes the period of uncleanness the same length with the menstrual days of the separation. See Leviticus 15:19.