2,101. Resetting Her Days
Hilchos Issurei Biah 8:17
Here’s what a woman must do in order to reset her niddah days so that she’ll know if she’s definitely a zavah or if there’s a doubt about that, as well as to know when her zivah days begin. It all depends on how many days she sees blood. If she sees blood for one or two days, she counts the rest of the seven, followed by the eleven days that follow them.
Hilchos Issurei Biah 8:18
If she sees blood for three days, there’s a doubt as to whether or not she’s a zavah; maybe one of these days preceded her niddah days and the other two were at the start of her niddah days. The same is true if she saw blood for four days: maybe two days preceded her niddah days and two were at the start of her niddah days. She must therefore keep five as the rest of her seven (niddah days) and follow them with her eleven (zivah) days.