Niddah, Chapter 9, Mishnah 9. This mishnah turns to the subject of the veses, the fixed pattern by which a woman's period announces itself, and it teaches that such a pattern can be built not only out of dates on a calendar but out of the physical sensations that regularly accompany her cycle.
The mishnah opens: "Kol isha sheyeish lah veses, dayah sha'atah" - any woman who has a set pattern accompanying her menstruation, her moment is enough for her. The practical meaning is this: when she sees blood, the tumah begins from that moment and no earlier. Whatever she touched and whatever she handled beforehand is not thrown into doubt, and there is no concern that tumah spreads backward in time to the objects that passed through her hands earlier. From the point of seeing onward, what she touches becomes tamei; before that point, her handling of pure items stands undisturbed.
Which signs qualify? The mishnah introduces its list with the words "Ve'eilu hein havestos", presenting the symptoms that can serve as a woman's fixed pattern. "Mefahekes": yawning, meaning that the onset of her period is regularly heralded by yawning. "Me'ateshes": sneezing that comes upon her when she menstruates. "Vechosheshes bpi kereisah": an ache in the upper opening of her belly, "uveshiplei mei'eha": and pain low down in her abdomen. "Veshofa'as": a mucous secretion. "Uch'min tzemarmoros ochazin osah": or a shivering chill that grips her. "Vechein kayotzei bahen": and other things of this sort. In other words, these examples are illustrations rather than a fixed roster, and any bodily indication of the same type that dependably arrives along with her period is included.
How does such a sign acquire legal standing? "Vechol shekav'ah lah shloshah pe'amim, harei zeh veses" - whatever she has established three times is a veses. Three consecutive periods in which the bleeding arrived together with that same symptom are what fix the pattern. Once the pattern is fixed, the symptom is no longer a private curiosity of her body; it is a recognized veses, and it carries with it the halachic consequence with which the mishnah began, that her moment of seeing is sufficient and nothing is rendered tamei retroactively.