20נִדָּה שֶׁבָּדְקָה עַצְמָהּ בְּתוֹךְ יְמֵי נִדָּתָהּ וּמָצְאָה שֶׁפָּסַק הַדָּם וַאֲפִלּוּ פָּסַק בְּשֵׁנִי לְנִדָּתָהּ וְשָׁגְגָה אוֹ הֵזִידָה וְלֹא בָּדְקָה עַד לְאַחַר נִדָּתָהּ יָמִים רַבִּים וּכְשֶׁבָּדְקָה מָצְאָה טֻמְאָה. אֵין אוֹמְרִים שֶׁמָּא כָּל אוֹתָן הַיָּמִים הָיְתָה טְמֵאָה וְתִהְיֶה זָבָה אֶלָּא כָּל אוֹתָן הַיָּמִים שֶׁלֹּא בָּדְקָה בְּחֶזְקַת טָהֳרָה. בָּדְקָה עַצְמָהּ וּמָצְאָה טְמֵאָה. אֲפִלּוּ בָּדְקָה בַּשְּׁבִיעִי לְנִדָּתָהּ וּבֵין הַשְּׁמָשׁוֹת לֹא בָּדְקָה עַצְמָהּ כְּדֵי לִפְרשׁ מִטֻּמְאַת נִדָּה אֶלָּא הִמְתִּינָה יָמִים וְאַחַר כָּךְ בָּדְקָה וּמָצָאת טְהוֹרָה הֲרֵי זוֹ סְפֵק זָבָה. וְאִם מָצְאָה טְמֵאָה הֲרֵי זוֹ זָבָה וַדָּאִית שֶׁכֵּיוָן שֶׁבַּתְּחִלָּה מָצְאָה טְמֵאָה וּלְבַסּוֹף טְמֵאָה הֲרֵי זוֹ בְּחֶזְקַת שֶׁלֹּא פָּסַק. וְיוֹם רִאשׁוֹן שֶׁל נִדָּה אַף עַל פִּי שֶׁמָּצְאָה בּוֹ טְהוֹרָה הֲרֵי זוֹ כְּמִי שֶׁמָּצְאָה טְמֵאָה. שֶׁיּוֹם רִאשׁוֹן כֻּלּוֹ הֻחְזַק הַמַּעֲיָן פָּתוּחַ:
When a niddah inspects herself in the midst of her days of niddah and discovers that her bleeding has ceased, even if it ceased on the second day of her menstrual period, and either, inadvertently or intentionally, did not inspect herself again until many days after her [days of] niddah and discovers impurity, we do not say that she was impure for all those days and [hence,] she is a zavah. Instead, throughout the entire time that she did not inspect herself, we operate under the presumption that she is pure. If she inspected herself and found impurity, even if she checked herself on the seventh of her days of niddah, if she did not inspect herself again [before] bein hashamashot to separate herself from the impurity of niddah, but instead waited [several] days and afterwards inspected herself and found that she was pure, there is an unresolved doubt if she is considered a [major] zavah. If she discovered impurity, she is definitely a zavah. [The rationale is that since] at the beginning she discovered impurity and at the end, she discovered impurity, we operate under the presumption that she did not stop bleeding. On the first day of menstruation, even though a woman [conducted an inspection and] found that she was pure, it is as if she discovered impurity. For on the first day of menstruation, we operate under the presumption that a woman's flow will continue.