Niddah, Chapter 4, Mishnah 6. This mishnah is a direct continuation of what the previous mishnah taught about a woman who is in difficult labor, a makashah, a woman whose childbirth is accompanied by pain. The blood that comes together with those labor pains does not carry the ordinary status of blood that a woman sees. It is dam leida, blood of childbirth, and halacha treats it as its own category rather than as dam niddah or dam zivah.
That naturally raises a question, and our mishnah opens with it: "Kamah hi kishuyah?" How long can this stage of kishuy last? In other words, how far ahead of the birth may we still attribute a woman's pains, and the blood that accompanies them, to the childbirth itself, so that the blood is classified as dam leida and not as ordinary dam niddah or dam zivah? Three opinions are recorded.
Rabbi Meir: forty or fifty days
"Rabbi Meir omer: afilu arba'im vechamishim yom." Rabbi Meir holds that even forty or even fifty days before the birth, pains can already be pains of childbirth, and the blood that comes with them is dam leida. The Gemara explains the two numbers: forty days is the measure for a woman who is healthy and strong, while fifty days is the measure for a woman who is weak or ailing, whose body begins to feel the strain of the coming birth earlier. Either way, according to Rabbi Meir the window is a wide one.
Rabbi Yehudah: the month of the birth
"Rabbi Yehudah omer: dayah chodshah." Rabbi Yehudah limits the window considerably. It is enough to allow her the month of the birth itself, meaning the ninth month of the pregnancy. Once she has entered that month, we assume that pains she experiences are the pains of the approaching birth, and blood that accompanies them is dam leida. In any earlier month we make no such assumption.
Rabbi Yose and Rabbi Shimon: two weeks
"Rabbi Yose veRabbi Shimon omrim: ein kishuy yoser mishtei shabbasos." Rabbi Yose and Rabbi Shimon narrow the window further still: the stage of kishuy extends no more than two weeks. Only within the final fortnight before the birth do we say that her pains belong to the childbirth, so that the blood which comes with them is dam leida.
The halacha
The accepted halacha follows the view of Rabbi Yose and Rabbi Shimon, and the difference this makes in practice is considerable. Picture a woman who is still three or four weeks away from her due date, who feels contractions that are genuine to her, and who sees blood together with them. That sighting cannot be classified as dam leida. Instead, where she stands in her cycle determines everything: if the blood appears during the stretch of days that renders her a niddah, she is a niddah, and if it appears during her days of zivah, what she has seen is dam zivah. Nor does it help if the baby arrives right on the heels of those very pains. Since the blood was seen before the two week period had begun, it never takes on the status of blood of childbirth.
So the mishnah has given us a measured definition of a concept that could easily have been left vague. The category of dam leida is real, but it has boundaries, and by fixing those boundaries at two weeks the halacha tells us precisely when a woman's pain reclassifies her blood and when it does not.