Leviticus 15 is concerned with normal and abnormal, male and female genital discharges. As scholars have noted, the chapter is structured chiastically: (A) Verses 2-15 describe males with abnormal (gonorrhea) discharges (zav), (B) verses 16-18 describe men with normal (“nocturnal”) discharges (qeri), (B’) verses 19-24 describe women with normal (menstrual) discharges (niddah), and (A’) verses 25-30 describe women who experience abnormal discharges (zavah). Both women and men who experience “normal” discharges (B, B’) are not required to perform elaborate purification rituals, nor must they wait any period of time free of discharge before becoming pure. However, women and men who experience “abnormal” discharges (A, A’) must perform elaborate purification rituals and wait seven days following the cessation of their discharges before they can become pure.
In addition to the biblical distinction between normal and abnormal discharges, the rabbis developed two more categories within the preexisting class of zav/zavah; namely, the “major” and “minor” zav/ah (zav/ah gedol/ah and zav/ah qeta/nah). Regarding the two different types of abnormal female discharges, the “major zavah” is a woman who experiences three discharges on three consecutive days during the “eleven days between menstrual periods,” otherwise known as “the days of zivah.” The “minor zavah,” refers to a woman who, at some point during that same time-span, experiences a flow of just one or two days. Only a “major zavah” is required to fully observe the rites as described in category A’. However, a “minor zavah” needs merely to “look out” for further contamination on the following day (shomeret yom ke-neged yom). Once she washes herself, she is considered pure at sundown.
In sum, the rabbinic system contains three different sets of rules for women who menstruate. For example, a.) at the onset of her first period, a woman is rendered a niddah and must count seven days before purifying herself through immersion in a ritual bath (miqveh). Even if blood continues to appear for the duration of the seven days, she is still permitted to purify herself at the conclusion of the week provided that her discharge ceases before the eighth day (upon which she immerses). b.) If blood continues to appear on the eighth day, or alternatively, if she purifies herself but then discovers that she is menstruating any time within the next eleven days, she is considered to be a “minor zavah,” and she must examine herself to make sure that she is not still menstruating. She then may immerse in a miqveh on the following day. The same rule applies if she menstruates for a second consecutive day. c.) However, if she sees blood for three consecutive days, she is considered to be a “major zavah,” and cannot begin counting seven days to purification, i.e. “seven clean days,” until she completely ceases to menstruate.
Shai Secunda, “‘Dashtana – “Ki Derekh Nashim Li”‘: A Study of the Babylonian Rabbinic Laws of Menstruation in Relation to Corresponding Zoroastrian Texts,” (Ph.D. diss., Yeshiva University, 2007), 142-144.