יורה דעה ק״צYoreh De'ah 190

Se’if 1א׳

דְּבַר תּוֹרָה אֵין הָאִשָּׁה מִטַּמְּאָה וְלֹא אֲסוּרָה לְבַעֲלָהּ עַד שֶׁתַּרְגִּישׁ שֶׁיָּצָא דָּם מִבְּשָׂרָהּ, וַחֲכָמִים גָּזְרוּ עַל כֶּתֶם שֶׁנִּמְצָא בְּגוּפָהּ אוֹ בִּבְגָדֶיהָ, שֶׁהִיא טְמֵאָה, וַאֲסוּרָה לְבַעֲלָהּ אֲפִלּוּ לֹא הִרְגִּישָׁה, וַאֲפִלּוּ בָּדְקָה עַצְמָהּ וּמָצְאָה טְהוֹרָה. וּצְרִיכָה הֶפְסֵק טָהֳרָה, שֶׁתִּבְדֹּק עַצְמָהּ וְתִמָּצֵא טְהוֹרָה, וְאַחַר כָּךְ תִּמְנֶה שִׁבְעָה נְקִיִּים מִיּוֹם הַמְּצִיאָה כְּאִלּוּ רָאֲתָה וַדַּאי, וּכְמוֹ שֶׁיִּתְבָּאֵר לְקַמָּן סִימָן קצ"ו. וְאִם הִרְגִּישָׁה שֶׁנִּפְתַּח מְקוֹרָהּ לְהוֹצִיא דָּם וּבָדְקָה אַחַר כָּךְ וְלֹא מָצְאָה כְּלוּם, יֵשׁ מִי שֶׁאוֹמֵר שֶׁהִיא טְמֵאָה.

According to Torah law, a woman is not rendered teme'ah and is not forbidden to her husband until she feels a sensation that blood left her body, but the sages decreed that upon seeing a blood stain on her body or clothing that she is teme'ah and forbidden to her husband even though she did not feel the blood leave her body, and even if she examines herself [afterwards] and finds herself clean. She requires a hefsek tahara to examine herself and find she is clean, and afterwards to count seven clean days from the day of her examination, as though she had certainly seen [niddah blood] as will be explained later in section 196. And if she felt that her uterus opened to release blood but checked herself afterward and did not find anything, there are those who say she is still teme'ah.

Se’if 2ב׳

לֹא גָּזְרוּ בְּתִינֹקֶת שֶׁלֹּא הִגִּיעַ זְמַנָּהּ לִרְאוֹת, דְּהַיְנוּ שֶׁהִיא פְּחוּתָהּ מִי"ב, אֲפִלּוּ הֵבִיאָה שְׁתֵּי שְׂעָרוֹת, וְכֵן אֲפִלּוּ הָיְתָה יְתֵרָה מִי"ב אִם בְּדָקוּהָ וְלֹא הֵבִיאָה ב' שְׂעָרוֹת, בֵּין שֶׁהִיא בְּתוּלָה בֵּין שֶׁהִיא בְּעוּלָה, וַאֲפִלּוּ אִם רָאֲתָה כְּבָר ב' פְּעָמִים. אֲבָל לְאַחַר שֶׁרָאֲתָה שָׁלֹשׁ פְּעָמִים חוֹשֶׁשֶׁת לְכֶתֶם.

They did not decree in the case of a young girl whose time to see has not come, that is, that she is less than twelve years, even if she has brought forth two hairs; and likewise even if she was more than twelve, if they examined her and she has not brought forth two hairs, whether she is a virgin or one who has had intercourse, and even if she has already seen two times. But after she has seen three times she is concerned for a stain.

Se’if 3ג׳

הָיְתָה שׁוֹפַעַת כַּמָּה יָמִים, אוֹ שֶׁהָיְתָה מְזַלֶּפֶת טִיף אַחַר טִיף בְּלֹא הֶפְסֵק, אֵינוֹ אֶלָּא כִּרְאִיָּה אַחַת עַד שֶׁתַּפְסִיק. אֲבָל אִם פָּסְקָה מְעַט, וְחָזְרָה וְרָאֲתָה שָׁלֹשׁ פְּעָמִים אֲפִלּוּ בְּיוֹם אֶחָד, הֲרֵי זוֹ מֻחְזֶקֶת בְּדָמִים וְכִתְמָהּ טָמֵא. וְיֵשׁ מִי שֶׁאוֹמֵר שֶׁאֵין כִּתְמָהּ טָמֵא אֶלָּא אִם כֵּן רָאֲתָה דָּם שְׁלֹשָׁה וְסָתוֹת. וְיֵשׁ לְהַחְמִיר כַּסְּבָרָא הָרִאשׁוֹנָה. (הִיא סְבָרַת רַשִׁ"י וְרַשְׁבָּ"א וְהַטּוּר).

She was flowing several days, or she was dripping drop after drop without interruption — it is only as one sighting, until she ceases. But if she ceased a little, and went back and saw three times, even in one day, this one is established as one subject to blood, and her stain is unclean. And there is one who says that her stain is not unclean unless she saw blood at three vesatot (the fixed time at which a woman's period comes). And there is ground to be stringent in accordance with the first reasoning. (it is the reasoning of Rashi and the Rashba and the Tur)