Leviticus 15 (text is found below) distinguishes between a woman who discharges blood during her menstrual period (vs. 19-24) and a woman who discharges blood not during her menstrual period (vs. 25-30). There are rules (these will get a bit more complicated) that distinguish between the two. Here they are briefly: When a woman first establishes her menstrual cycle, the blood is considered to be menstrual blood. She is now impure for seven days, no matter how many days she menstruates. As long as she stops bleeding by the seventh day she goes to the mikveh at the end of the seventh day and she is pure (actual practice differs, as we shall discuss later). After seven days she now enters what are called "the days of zivah" meaning non-menstrual blood. If she bleeds on one of these days, she at first will observe one day of purity for every day of blood. So if she bleeds on the eighth day, she will go to the mikveh on the ninth day and observe purity for that day and then if she sees no more blood, she will be pure in the evening. The same is true if she sees blood on the ninth day as well. She will go to the mikveh on the tenth day and is pure at night. However, if she sees blood for three days in a row then she will need to go through "seven clean days" where she does not discharge blood. After seven clean days she goes to the mikveh and then brings a sacrifice and she is pure. According to rabbinic calculation there are eleven days of "zivah" between a woman's menstrual period and her next period. During these eleven days, if she discharges blood it is considered "zivah" (non-menstrual blood) and she observes one day of purity for each day or seven days if she bleeds three straight days. After these eleven days are completed, she begins a seven day period during which any blood is considered menstrual blood. This pattern will then repeat itself. The above rules are according to the Torah's rules. Late in the tannaitic period some Jews began to act more stringently and treat all blood as if it was zivah. Functionally what this means is that a woman will always observe seven clean days after the end of her menstrual period. So, according to Torah law if a woman has menstrual blood for five days she would be clean after the seventh day. But according to post-talmudic practice, she will need to wait seven more days and will be clean only on the 12th day. Tractate Niddah will also deal with the blood discharged by a woman after childbirth. An important issue to remember while learning this tractate is that the practical significance of the laws of niddah was much broader than it is today. The mishnah portrays a world in which people observe the full array of purity laws. A woman who was unclean due to menstruation would defile terumah and hallah and other holy foods. Today, we think of these laws as limited to the permissibility of sexual relations between a man and a woman. While this was certainly an issue in mishnaic times as well, it is important to remember that it wasn't the only issue. Finally, a note on my treatment of this material. A woman's menstruation can be a sensitive and personal issue. I will try to keep a "clinical" tone throughout my commentary, avoiding both apologetics and criticism. I believe that it is important for people to understand the technicalities of this system, before they attempt to delve into any greater socio-religious meaning. I also think that the rabbinic discussion of the material is in itself clinical, or perhaps mathematical. The rabbis are intensely interested in the intricacies of halakhah and spend little time discussing how such issues would affect a woman's life. This is simply the nature of the discussion. Below are most of the verses relevant to the subject of menstrual and non-menstrual blood. We will make frequent reference to them throughout the commentary.
Leviticus Chapter 12 1 The Lord spoke to Moses, saying: 2 Speak to the Israelite people thus: When a woman at childbirth bears a male, she shall be unclean seven days; she shall be unclean as at the time of her menstrual infirmity. — 3 On the eighth day the flesh of his foreskin shall be circumcised. — 4 She shall remain in a state of blood purification for thirty-three days: she shall not touch any consecrated thing, nor enter the sanctuary until her period of purification is completed. 5 If she bears a female, she shall be unclean two weeks as during her menstruation, and she shall remain in a state of blood purification for sixty-six days.
Leviticus Chapter 15 6 On the completion of her period of purification, for either son or daughter, she shall bring to the priest, at the entrance of the Tent of Meeting, a lamb in its first year for a burnt offering, and a pigeon or a turtledove for a sin offering. 7 He shall offer it before the Lord and make expiation on her behalf; she shall then be clean from her flow of blood. Such are the rituals concerning her who bears a child, male or female. 8 If, however, her means do not suffice for a sheep, she shall take two turtledoves or two pigeons, one for a burnt offering and the other for a sin offering. The priest shall make expiation on her behalf, and she shall be clean. 19 When a woman has a discharge, her discharge being blood from her body, she shall remain in her impurity seven days; whoever touches her shall be unclean until evening. 20 Anything that she lies on during her impurity shall be unclean; and anything that she sits on shall be unclean. 21 Anyone who touches her bedding shall wash his clothes, bathe in water, and remain unclean until evening; 22 and anyone who touches any object on which she has sat shall wash his clothes, bathe in water, and remain unclean until evening. 23 Be it the bedding or be it the object on which she has sat, on touching it he shall be unclean until evening. 24 And if a man lies with her, her impurity is communicated to him; he shall be unclean seven days, and any bedding on which he lies shall become unclean. 25 When a woman has had a discharge of blood for many days, not at the time of her impurity, or when she has a discharge beyond her period of impurity, she shall be unclean, as though at the time of her impurity, as long as her discharge lasts. 26 Any bedding on which she lies while her discharge lasts shall be for her like bedding during her impurity; and any object on which she sits shall become unclean, as it does during her impurity: 27 whoever touches them shall be unclean; he shall wash his clothes, bathe in water, and remain unclean until evening. 28 When she becomes clean of her discharge, she shall count off seven days, and after that she shall be clean. 29 On the eighth day she shall take two turtledoves or two pigeons, and bring them to the priest at the entrance of the Tent of Meeting. 30 The priest shall offer the one as a sin offering and the other as a burnt offering; and the priest shall make expiation on her behalf, for her unclean discharge, before the Lord.