Leviticus 15:30

And the priest shall offer the one for a sin-offering and the other for a burnt offering; and the priest shall make an atonement for her before the Lord For the issue of her uncleanness. While ordinary menstruation required no sacrifice, this abnormal flow, being an uncleanness which estranged from God, made an offering of atonement necessary, for the Lord wanted external, physical purity as a mark and expression of internal cleanness in all the members of His people.