סימן קנה סעיף ח
אִשָּׁה שֶׁיֵשׁ לָהּ וֶסֶת קָבוּעַ, אַף אִם הִגִּיעַ פַּעַם אַחַת אוֹ שְׁתֵּי פְעָמִים יוֹם וֶסְתָּהּ אוֹ סִימָן וֶסְתָּהּ וְלֹא רָאֲתָה, אַף עַל פִּי שֶׁבָּרוּר לָהּ בְּבֵרוּר גָּמוּר שֶלֹּא רָאֲתָה, כְּגוֹן שֶהָיָה לָהּ כָּל מֶשֶׁךְ הַוֶּסֶת מוֹךְ דָּחוּק, מִכָּל מָקוֹם עֲדַיִן לֹא נֶעֱקַר וֶסְתָּה, וּצְרִיכָה עֲדַיִן לָחוּשׁ גַם לַפַּעַם הַשְּׁלִישִית. אֲבָל אִם הִגִּיעָה גַּם הַפַּעַם הַשְּׁלִישִׁית וְלֹא רָאֲתָה, אִם בָּרוּר לָהּ בְּבֵרוּר גָּמוּר שֶלֹּא רָאֲתָה בְּכָל שְׁלשֶׁת הַפְּעָמִים, כְּגוֹן שֶׁבְּכָל פַּעַם הָיָה לָּהּ מוֹךְ דָּחוּק כָּל מֶשֶׁךְ הַוֶּסֶת, מֵעַתָּה נֶעֱקַר וֶסְתָּהּ, וְאֵינָהּ צְרִיכָה לָחוּשׁ לוֹ עוֹד, כִּי כָל וֶסֶת נֶעֱקָר בְּשָׁלֹשׁ פְּעָמִים, אֲפִלּוּ הָיָה קָבוּעַ כַּמָּה שָׁנִים. אֲבָל וֶסֶת שֶׁאֵינוֹ קָבוּעַ, דְּהַיְנוּ רְאִיָה שֶלֹּא קְבָעַתָּהּ רַק פַּעַם אַחַת אוֹ שְׁתֵּי פְעָמִים, אִם הִגִיעַ פַּעַם אַחַת הַיוֹם הַזֶה אוֹ הַסִימָן הַזֶּה וְלֹא רָאֲתָה, וְאַף עַל פִּי שֶׁלֹּא בָדְקָה אֶת עַצְמָהּ, אֶלָּא שֶׁלֹּא הִרִגִּישָׁה שׁוּם רְאִיָה, שׁוּב אֵינָהּ צְרִיכָה לָחוּשׁ לָהּ, כִּי כָל מַה שֶׁלֹּא נִקְבַּע בְּשָׁלֹשׁ פְּעָמִים, נֶעֱקָר בְּפַעַם אַחַת, וַאֲפִלּוּ בְּלֹא בְדִיקָה.
A woman who has a fixed period, even if once or twice the date of her period (arrived), or her characteristic symptoms (arrived), and she did not perceive blood, even though she is certain beyond any doubt, that she did not perceive, for instance if during the entire period she had a cloth [or tampon] packed tightly in place,11The Kitzur Shulchan Aruch rules like the Chavas Da’as who requires such a tampon in place throughout the entire onah. However, this is not the prevailing custom anywhere. The Chazon Ish 80:22 says she has to examine herself [only] one time during that onah. It is better to check once in the beginning and again at the end of the onah. still her regular period is not eliminated, and she must still be mindful of it even the third time. But if the third time also arrived and she did not perceive anything, if she is absolutely certain that she did not perceive anything on any of these three times; for instance, if at each time she had a cloth packed tightly into place throughout the date of her expected period, from then on her regular period has been eliminated, and she need not be concerned about it anymore. For every regular fixed period is eliminated after failing to happen three times, even if it was regular for many years. But a period which is not established, such as a perception that occured only once or twice, then if this day arrived, or this particular sign arrived one time, and she did not perceive her menstrual flow, even if she did not examine herself, as long as she did not feel anything, she no longer needs to observe this period. Whatever has not been established by three consecutive occurrences is eliminated by one time, even without examination.
תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)