Niddah, Chapter 2, Mishnah 1. In the opening chapter we learned of four types of women about whom the Sages ruled dayah sha'atah, that the moment of the sighting suffices: the besulah, a girl who has never yet menstruated; the me'ubberes, who is carrying a child; the meinekes, who is nursing; and the zekeinah, who has passed the years of menstruation. Should one of these see blood, her tumah begins only from then and reaches nothing backward in time. Our Mishnah now warns that such a leniency is not a release from responsibility. It turns to the topic of bedikos, internal examinations, spelling out upon whom they are incumbent and at which times.
A lenient ruling is not a dispensation
"Af al pi she'amru dayah sha'atah, tzerichah lihyos bodekes." Although the Sages limited these women's tumah to the hour of the sighting alone, a woman is nonetheless obligated to keep checking herself. This applies to a woman who occupies herself with taharos, foods and vessels whose tahor standing must be preserved; she performs internal examinations so that her own status is known to her and nothing in her care is spoiled.
Two women for whom an examination accomplishes nothing
The Mishnah records two who stand outside this obligation. "Chutz min haniddah": a niddah is excluded, for her tumah is already an established fact, and a bedikah has nothing left to determine. Nor is she alone: "vehayosheves al dam tohar," a woman in the stretch of pure blood that follows childbirth, is likewise exempt. Recall that a mother is temei'ah for seven days after bearing a son and for fourteen days after bearing a daughter. Once those days pass, the era of dam tohar begins, thirty three days in the case of a boy and sixty six in the case of a girl, and whatever she may see throughout that time carries no tumah at all. Since a discovery of blood then would leave her tahor in any event, checking would accomplish nothing for her.
Examinations surrounding relations with her husband
"Umeshameshes be'edim." The same woman engaged with taharos employs edim, cloths used for examination, in connection with marital relations: one check beforehand and a second afterward, so that there is no doubt about her condition at that time.
Once again two are excluded: "Chutz miyosheves al dam tohar uvesulah shedameha tehorim." One is the mother during her days of pure blood after birth. The other is a girl who has not yet begun to menstruate, whose blood is treated as tahor, since it is not attributed to the uterus. In neither case would blood found on the cloth make her temei'ah, so there is no point in the examination.
The times of the daily bedikos
"Ufa'amayim tzerichah lihyos bodekes." A woman handling taharos checks herself on two occasions every day: "beshacharis uvein hashemashos," in the morning and again at twilight, once the sun has gone down. "Uvesha'ah she'overes leshamesh es beisah," and a further examination is made when she is about to have relations with her husband.
An added obligation for the wife of a kohen
"Yeseiros aleihen kohanos." One group carries a requirement beyond all of the above: kohanos, wives of kohanim who partake of terumah. "Besha'ah shehen ochlos biterumah": as she is about to eat terumah she performs a bedikah, establishing her taharah before she partakes of it.
"Rebbi Yehudah omer: af bisha'as avarasan mile'echol biterumah." Rebbi Yehudah holds that a check is needed at the conclusion of her eating as well. What troubles him is the remainder, which some other kohen may come and eat afterward. An examination when she is finished shows that what she has left over was not compromised and that the terumah still on hand is tahor.
The accepted ruling is not that of Rebbi Yehudah. We follow the Tanna Kamma: a kohenes examines herself before partaking of terumah, and no examination is demanded of her once she has finished.