Halakhah zu Wajikra 15:19

In order to understand the problem, we must first define the Biblical concepts of nidah and zavah.³For a full introduction to these concepts, see Rav Pinchas Kehati’s commentary to the Mishnah (Arachin 2:1) and Badei Hashulchan (Introduction to 183). Nidah refers to a woman who sees menstrual blood on a day when she expects it to flow (Vayikra 15:19). This flow renders her ritually impure and prohibits relations with her husband for seven days. Even if she continues to see blood all seven days, she may go to the mikvah and purify herself immediately after they end. On the other hand, if a woman sees uterine blood at an unexpected time, then she becomes a zavah. If the bleeding persists for three consecutive days, she must wait until all bleeding ceases. She then counts seven days before she may visit the mikvah (Vayikra 15:25-28). Thus, once a zavah sees blood for three consecutive days, the total time that she remains impure will always last at least three days longer than the seven-day nidah period.

The commandment of the matter of the impurity of the menstruant, that is impure and renders impure: That the menstruant be impure and render others impure, as it is stated (Leviticus 15:19), "and anyone who touches her shall be impure until the evening."

To not have intercourse with a menstruant woman: To not have intercourse with a woman when she is menstruant, as it is stated (Leviticus 18:19), "And to a woman in menstrual impurity, you shall not approach." And the time of her menstruation continues for seven days, as it is written (Leviticus 15:19), "seven days shall she be in her menstruation." And whether she [experiences blood] once during them or gushes all seven, she is a menstruant (Niddah 73a). And the entire time that she does not immerse (in a mikveh) - even after the seven - she is [considered] a menstruant; since the verse made it dependent upon days and immersion, as it is stated with impure people (Leviticus 15:18), "and they shall wash in water." And they, may their memory be blessed, said [that it is] a constructive paradigm (binyan av) to any impure one, that he is with his impurity until he immerses (see Rabbenu Bachya on Leviticus 15:19 in the name of the Geonim). And likewise, they, may their memory be blessed, also expounded (Shabbat 64b), "'Shall she be in her menstruation' - she shall be in her menstruation until she goes into water."