Commentary for Niddah 3:7

Mishnah: Shammai says, "All women, their time [of seeing blood] is enough" - If she sees blood, it is enough for us if she defiles terumah and taharot that she came in contact with from the time [of seeing blood] and onward, and we do not say that "just as she has now found blood in the outer chamber [of the uterus], if she had performed a bedikah [check] even earlier, she would have found blood as well, and the [only reason she did not see the blood before is because the] walls of her uterus held the blood in." And we have [already] established in the chapter "Yotetzeh Dofen" (Yevamot 40a) that all women become impure [for the laws of Niddah when blood is] in the outer chamber [of the uterus].

"From pekidah to pekidah" - [A case of where a woman, for example,] checked (performed a bedikah) herself on Sunday and found that she was ritually pure, and on Wednesday checked (performed a bedikah) and found that she was ritually impure. [In this case] we worry [that she was ritually impure even before she found blood] and declare ritually impure, out of doubt, all taharot that she came into contact with from the time of her bedikah on Sunday. For it is possible that as soon as she took her hand away [from the bedikah on Sunday] she "saw" (i.e. there was a flow of) blood, and the walls of her uterus held the blood in place.

"And even for many days"...between the first check (bedikah) and the second check (bedikah), we declare all taharot she handled during days in between (the first and second bedikot) to be ritually impure. However, those [taharot that were handled before the first bedikah are] ritually pure, since she checked herself (through a bedikah) and found herself to be ritually pure.

"Not like the words of this (Hillel)" - that is overly stringent, for the walls of the uterus would not hold [the blood] in for so many days.

"Not like the words of this (Shammai)" - that is overly lenient.

"Rather, a twenty four hour period can reduce (a longer than twenty four hour period) between two checks (i.e. bedikot)" - For if there are many days in between two bedikot, we do not declare all the taharot [handled] in the days between the two bedikot ritually impure, but just those [taharot] that she dealt with during the previous day at this hour (i.e. twenty four hours prior to her seeing blood). And the reasoning of the Rabbis is explained later on in the Gemarah. It appears that a twenty four hour period can reduce [a longer amount of] time in between two bedikot.

"...and a period in between two checks (i.e. bedikot) (that is less than twenty four hours) can reduce a twenty four hour period." - For if the time in between two bedikot is less than a twenty four hour period, we go after it (the smaller amount of time in between the two bedikot) in order to be lenient. For example, if she performed a bedikah in the morning and found herself to be ritually pure, and she again performed and bedikah in the evening and found herself to be ritually impure - [as is mentioned] later on in the Mishnah (Niddah 11a) "Sometimes she is required to perform a bedikah in the morning as well as at twilight..." - we do not declare the taharot of the previous night ritually impure, since she performed a bedikah in the morning and found herself to be ritually pure. And we cannot say that there was blood present at that time (i.e. the previous night). Rather, those taharot from time time [after] the morning bedikah and onward we make ritually impure - which is the time between the two bedikot.

"Every woman that has a regular time (i.e. menstrual cycle)" - It's set three times [in a row] (i.e. three regular menstrual cycles, one after the other), and she checks herself (i.e. performs a bedikah) at her time and finds herself ritually impure.

"...her hour (i.e. the time at which they see menstrual blood) is [a] sufficient (starting point for determining whether the terumot and taharot they handled are ritually impure)" - And [one] cannot say that it (i.e. the blood) was there orginally, for surely a guest [i.e. menstruation] comes on time.