Niddah, Chapter 4, Mishnah 5. Our Mishnah asks how to classify blood that a woman sees while she is in hard labor, and whether that blood should be attributed to the birth itself or to zivah.
The framework of niddah days and zivah days
Once a woman sees blood, the Torah places her in the status of niddah for a stretch of seven days. The length of the flow itself makes no difference: bleeding of one day, of two, or of three all yield the same seven day period of niddah tumah. When that week runs out her yemei niddah are behind her, and mid'oraisa the eleven days that follow are yemei zivah, a window in which the status of zavah is available to her. Should blood appear on three days in a row anywhere inside those eleven, she carries the status of zavah, and taharah arrives only after she counts shivah neki'im, a full week with no blood at all.
Blood seen during labor
The Mishnah begins: "Hamakshah, niddah." A woman who is having difficult labor pains, who is in the midst of childbirth, and who sees blood during her seven niddah days: that blood renders her a niddah in the ordinary way.
Now picture that same woman at a later point. Her niddah week has run its course, and she stands inside the eleven day stretch in which zivah applies. In the words of the Mishnah: "Kishesahasah sheloshah yamim besoch achad asar yom." Her labor dragged on across three successive days that fell within those eleven, with blood appearing throughout.
Had she been in labor only for twelve hours, or even for a day or two, the blood would simply be attributed to the birth, dam leidah. But three consecutive days of bleeding within the yemei zivah raises a real question: is this in fact dam zavah, or is it still the blood of a woman giving birth? The test the Mishnah supplies is whether the labor pains themselves continued without interruption.
The dispute of Rebbi Eliezer and Rebbi Yehoshua
"Veshafsah mei'eis le'eis veyaldah, harei zo yoledes bezov, divrei Rebbi Eliezer." Suppose her contractions let up for a stretch of twenty four hours, and only afterward did the delivery take place. Rebbi Eliezer holds that she is a yoledes bezov: her child arrived while she carried the status of zavah. The lull in the pains shows, looking backward, that the seventy two hours of flow which preceded it had nothing to do with the delivery. What she saw was dam zavah.
"Rebbi Yehoshua omer, laylah veyom keleilei Shabbos veyomo." For Rebbi Yehoshua the gap must be wider before we reclassify what she saw from birth blood into zivah blood. He demands a night together with the day that follows it, on the pattern of Shabbos, which opens on Friday evening and runs through Shabbos day, the two together forming one whole calendar day. If the respite from her contractions fell short of such a full day and the delivery then followed, what she saw is treated as plain dam leidah and no zivah status attaches. But if a complete calendar day of quiet did separate the contractions from the delivery, given that she had already seen blood on those three successive days, we understand in hindsight that it was dam zavah, and she is a yoledes bezivah.
The practical consequence is significant. A yoledes bezivah, after completing her yemei tumah, must count shivah neki'im, seven clean days, before her yemei taharah can begin and before blood she sees can be considered dam tohar.
What exactly must stop
The Mishnah closes by clarifying the nature of the required pause: "Sheshafsah min hatza'ar velo min hadam." The interruption we are measuring is a break from the labor pains. It is not a requirement that the bleeding itself cease during that time. What proves that the blood was not birth blood is the cessation of the pain of labor.