Mordechai Moshe Gelernter asked:

In the case of 2 days on and 2 days off. We say "shimur" is on day 19. If we say the woman can not become a Zavah Gedolah, why do we not say shimur on day 17 or 18, but only day 19?

Mordechai Moshe Gelernter, US of A

The Kollel replies:

Day 17 and 18 she is Tamei because she saw on those days (1-2, 5-6, 9-10, 13-14, 17-18). The day of the Re'iyah she is Tamei no matter what. The question is the day after, which in general she must be Shomeres (as a Zavah Ketanah). The simple reason is that we are afraid that she will see and become a Zavah Gedolah, which would mean that she was actually Tamei from the time of the first Re'iyah. Day 19 is actually not from the Yemei Zivah, but this Tana nevertheless says that she must be Shomeres the day after the Re'iyah even though she can no longer become a Zavah Gedolah; still, she must keep the extra day for there is a fear that she will see.

D. Zupnik