Halakhah su 'Arakhin 2:9

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.