Mishnayos Niddah Perek 10 Mishnah 2

In the case of a menstruating woman who examined herself on the seventh day of menstruation in the morning and found that she is ritually pure and eligible to immerse in a ritual bath that evening, but during twilight of the seventh day she did not perform an examination that marks the transition between the days when she has a flow of blood and the days when she no longer has a flow of blood but immersed despite not having performed the examination, and after several days she examined herself and found that she is ritually impure, the presumptive status of that woman is one of ritual purity from the time of her immersion until her examination, and all ritually pure items that she handled in the interim remain pure. If she examined herself on the seventh day of menstruation in the morning and found that she is ritually impure, i.e., her menstrual flow continued, and during twilight of the seventh day she did not perform an examination to confirm the transition from ritual impurity to ritual purity but immersed nonetheless, and after several days she examined herself and found that she is ritually pure, the presumptive status of that woman is one of ritual impurity from the time of her immersion until her examination, and all ritually pure items that she handled in the interim are impure. Since she found blood during her last examination in her days of menstruation, the concern is that the flow of blood continued during the days that followed, and therefore her immersion on the eve of the eighth day was ineffective. In a case where there was no blood found during the examination on the seventh morning and she did not examine herself during twilight, and several days later she discovered blood, where the mishna says that a woman’s presumptive status is one of ritual purity, that is the halakha only for the days following immersion. But she transmits ritual impurity to the ritually pure items that she handled before the examination in which she found blood for a twenty-four-hour period and from examination to examination, in accordance with the halakha of a woman who experiences bleeding (see 2a). And if she has a fixed menstrual cycle, on the day that she examined herself and found blood, her time is sufficient, i.e., it is assumed that the bleeding began then, and she does not transmit impurity retroactively. And Rabbi Yehuda says: With regard to any woman who did not perform the examination marking her transition from ritual impurity to ritual purity on the seventh day from minḥa time onward, even if she performed an examination and found no blood that morning, the presumptive status of that woman is one of ritual impurity. And the Rabbis say: Even if on the second day of her menstruation she performed the examination and found that she is ritually pure, and she did not perform the examination marking her transition from ritual impurity to ritual purity on the seventh day during twilight, and after several days she examined herself and found that she is ritually impure, the presumptive status of that woman is one of ritual purity from the time of her immersion until her examination.