Mishnayos Niddah Perek 4 Mishnah 7

For all the eleven days of ziva that follow the seven days of menstruation, a woman has the presumptive status of ritual purity, as it is unusual for her to experience bleeding on these days. If a woman sat and did not examine herself every morning and evening to determine whether she emitted blood and is impure, it makes no difference whether she failed to examine herself unwittingly or due to circumstances beyond her control, or even if she acted intentionally and did not examine herself; she remains ritually pure. She is rendered impure only if she examined herself and was found to have emitted blood. By contrast, if the time of her menstrual cycle arrived, when she is required to examine herself, and she did not examine herself, that woman is ritually impure, as it is typical for a woman to discharge blood at that time. Rabbi Meir says: If a woman was in hiding from danger, and the time of her menstrual cycle arrived and she did not examine herself on that day, that woman is pure, because fear drives away blood. There is therefore no concern that she might have emitted blood. But with regard to the seven clean days of the zav and the zava, and with regard to a woman who observes a clean day for a day she experiences a discharge during her days of ziva, if she fails to examine herself on those days, these women have a presumptive status of ritual impurity, as they already experienced a discharge.

כָּל אַחַד עָשָׂר יוֹם בְּחֶזְקַת טָהֳרָה. יָשְׁבָה לָהּ וְלֹא בָדְקָה, שָׁגְגָה, נֶאֶנְסָה, הֵזִידָה וְלֹא בָדְקָה, טְהוֹרָה. הִגִּיעַ שְׁעַת וִסְתָּהּ וְלֹא בָדְקָה, הֲרֵי זוֹ טְמֵאָה. רַבִּי מֵאִיר אוֹמֵר, אִם הָיְתָה בְמַחֲבֵא וְהִגִּיעַ שְׁעַת וִסְתָּהּ, וְלֹא בָדְקָה, הֲרֵי זוֹ טְהוֹרָה, מִפְּנֵי שֶׁחֲרָדָה מְסַלֶּקֶת אֶת הַדָּמִים. אֲבָל יְמֵי הַזָּב וְהַזָּבָה וְשׁוֹמֶרֶת יוֹם כְּנֶגֶד יוֹם, הֲרֵי אֵלּוּ בְּחֶזְקַת טֻמְאָה: