We learned in the opening Mishna of Massekhet Nidda (see above, daf 2), that according to Hillel and the Sages, a woman who does not have a set menstrual cycle (veset kavu’a), cannot assume that her status as a nidda begins at the time that she sees menstrual blood. Rather, she must assume that everything with which she came into contact in the last 24 hours or since the last time that she examined herself for purity will be rendered unclean.
The Mishna on today’s daf teaches that there are exceptions to this rule. Rabbi Eliezer teaches that there are four types of women for whom it suffices for them to reckon their period of uncleanness from the time of their discovering menstrual blood: a virgin, a woman in pregnancy, a nursing woman, and an old woman.
In explanation for these exceptions, the Mishna Aḥarona suggests that since these woman do not ordinarily menstruate, even though they did see menstrual blood on this occasion the Sages did not apply the concern lest the vaginal walls held in the menstrual blood for a time, since that is only a Rabbinic concern and the Sages do not establish rabbinic decrees in unusual situations. Furthermore, in these cases, each of the women has a well-established ḥezkat tahara – a presumption of purity – based on the fact that she does not ordinarily menstruate. This is not true of other women who menstruate on a regular basis.
A related approach offered by the Kehilat Ya’akov argues that in the case of other women, even if we do not have reason to suspect that at this particular instance of menstruation the vaginal walls held back the menstrual blood, making her ritually unclean retroactively, nevertheless it is reasonable to assume that at some point in her lifetime that will occur, and the Sages established a stringent approach due to the severity of the laws of ritual purity. In the case of these four women, however, since they do not ordinarily menstruate, we do not look at future incidents of menstruation, but only at the single case that is in front of us at this time. This specific incident does not call for a rabbinic decree.