MISHNA: Shammai says: For all women who do not have a fixed menstrual cycle, their time is sufficient. Women who discern that menstrual blood emerged do not need to be concerned that perhaps the flow of blood began before they noticed it. Rather, they assume their ritual impurity status begins at that moment, in terms of rendering impure teruma and ritually pure items with which they come in contact. Hillel says: From examination [mipekida] to examination, i.e., she assumes ritual impurity status retroactive to the last time she examined herself and determined that she was ritually pure, and this is the halakha even if her examination took place several days earlier. Any ritually pure item with which she came in contact in the interim becomes ritually impure.

And the Rabbis say: The halakha is neither in accordance with the statement of this tanna nor in accordance with the statement of that tanna; rather, the principle is: A twenty-four-hour period reduces the time from examination to examination. In other words, if her final self-examination took place more than twenty-four hours earlier, she need only concern herself with ritual impurity for the twenty-four-hour period prior to discerning the blood. And from examination to examination reduces the time from a twenty-four-hour period. In other words, if she examined herself in the course of the previous day and discovered no blood, she was certainly ritually pure prior to the examination.

For any woman who has a fixed menstrual cycle [veset], and she examined herself at that time and discovered blood, her time is sufficient, and it is only from that time that she transmits ritual impurity. And with regard to a woman who engages in intercourse while using examination cloths [be’edim] before and after intercourse, with which she ascertains whether her menstrual flow began, the halakhic status of such an action is like that of an examination, and therefore it reduces the time from a twenty-four-hour period, and reduces the time from examination to examination.

Her time is sufficient, how so? If the woman was sitting in the bed and engaged in handling ritually pure items, and she left the bed and saw blood, she is ritually impure and those items are ritually pure.

Although the Rabbis said that a woman without a fixed menstrual cycle transmits ritual impurity retroactively for a twenty-four-hour period, a woman with a fixed cycle counts her menstrual days only from the moment that she saw blood.

GEMARA: The mishna teaches that when a woman discerns that her menstrual blood emerged, Shammai holds that her impurity status starts from that moment, while Hillel maintains that it begins from the time of her most recent self-examination. The Gemara asks: What is the reason for the opinion of Shammai? The Gemara answers: Shammai holds that as there is a principle that when the status of an item is uncertain it retains its presumptive status, here too: Establish the woman in her presumptive status, and a woman remains with the presumptive status of being ritually pure. And what is the reasoning of Hillel? He claims that when one states the principle: Establish an item in its presumptive status, that applies only where there is no weakening in the presumptive status that is a result of the item itself. But in the case of a woman,