2,100. A Woman Who Doesn't Know Her Days
Hilchos Issurei Biah 8:15
A blind woman should check herself and give the cloth to a friend to examine. Women with congenital deafness or lacking mental faculties need to be checked by women with full faculties in order to establish fixed times for them, after which they are permitted to their husbands.
Hilchos Issurei Biah 8:16
Let’s say that a woman makes a mistake and doesn’t know when her niddah days begin. If she sees blood, she must be concerned about the possibility of being a zavah. Accordingly, if she saw blood for one or two days, she must wait the full seven days out of concern that she might have bled during her niddah days. If she saw blood for three days, she must count seven clean days, out of concern that she’s in her zivah days.