Niddah, Chapter 1, Mishnah 7. This chapter has been occupied with a single central question: when a woman discovers blood, from which moment is she considered temei'ah? As a rule, there is a retroactive element to her tumah, so that taharos she handled before she noticed anything can be affected. But the Mishnah earlier singled out four categories of women concerning whom the Sages ruled dayah sha'atah, "her moment is enough for her": they are temei'os only from the time they saw the blood and onward, and whatever they touched beforehand remains tahor. Our Mishnah now comes to define the limits of that leniency.
The four women
The four are: a besulah, a young girl who has not yet menstruated for the first time; a me'ubberes, a pregnant woman; a meinekes, a nursing woman; and a zekeinah, a woman past the age of menstruation. In each of these situations a sighting of blood is unexpected, and for that reason the Sages did not impose the usual retroactive tumah upon her.
Only the first sighting
The Mishnah asks: "Uvameh amru dayah sha'atah?" Regarding what case did the Sages say that her moment is sufficient, that she carries no retroactive tumah at all? The answer: "Bire'iyah rishonah," with respect to the first time each of these four women sees blood. That first appearance is treated as an isolated surprise, and so nothing that came before it is drawn into her tumah.
"Aval bashniyah," but once there is a second sighting, the picture changes. From the second occasion and onward, "mitam'ah me'es le'es," she conveys tumah retroactively for the preceding twenty four hours, and any taharos she came into contact with during that period become temei'os along with her. A second appearance of blood shows that her condition is no longer an anomaly, and she is therefore returned to the ordinary rule that applies to every woman.
When the first sighting came about through an external cause
The Mishnah adds an important qualification: "Ve'im ro'asah harishonah me'ones, af hashniyah dayah sha'atah." If that first sighting of blood came about me'ones, that is, it was not a natural flow arising from her own body but was brought on by some outside circumstance that forced it, then even the second sighting is governed by dayah sha'atah, and she is temei'ah only from that moment forward and not retroactively.
The logic is straightforward. The leniency of dayah sha'atah was granted precisely because these four women are not expected to see blood. A sighting produced by an external cause proves nothing about her natural condition, so it does not count as an established pattern. Consequently the second sighting is still, in effect, her first genuine one, and it too receives the leniency the Sages extended to these four women.