Shammai says: “For all women [who begin to menstruate] it suffices [to reckon their impurity from] the time [of their discovering it].” And Hillel says: “[Their impurity is reckoned backwards] from the [last] examination to the [previous] examination, even if this covers many days.” But the Sages say: “Neither according to the opinion of this one nor according to the opinion of this one, but [they are considered impure for] the past twenty four hours when this lessens the period from the [last] examination to the [previous] examination, and for the period from the [last] examination to the [previous] examination when this lessens the past twenty-four hours.” This mishnah discusses a woman’s ritual impurity as a menstruant. When a woman menstruates she makes impure certain things which she touches. The problem is that a woman may not always know when she begins to menstruate, in order to know which things that she touched are impure. Shammai is not concerned with this problem; he states that only things that they have touched after they discover that they are menstruating are impure. Hillel is very considered about this problem; he states that anything touched after the last time she checked herself is impure. In other words, since we cannot be certain when she began menstruating after the last time she checked, we are strict and anything from this time and onwards is impure. The Sages, who probably lived after Shammai and Hillel, found a compromise position. The things a menstruant touched before she discovered that she was menstruating are impure either within the last twenty-four hours or since she last checked, whichever was more recent. For example if it is now Tuesday afternoon and she checked herself on Tuesday morning, anything she touched between the morning and the afternoon is assumed to be impure. If however, it is now Tuesday afternoon and she checked herself Monday morning, anything she touched in the last twenty-four hour period is impure.
Any woman who has a regular period, it suffices [to reckon her impurity from] her set time. Any woman who has a regular cycle, begins to make things impure only from the moment she realizes that she is menstruating. It can be assumed that this woman did not begin menstruating at an earlier time since she has a regular cycle.
She who uses testing-cloths [when she has sexual relations], behold this is like an examination: it lessens either the period of the [past] twenty four hours or the period from the [last] examination to the [previous] examination. If a woman wipes herself clean after having relations with her husband, this counts as checking herself. If within the next twenty-four hours she should realize that she is menstruating, she has made impure only those things which she touched since she had relations and cleaned herself.