Niddah 11: Attempts to Develop a Predictable Menstrual Cycle

If a woman has a veset kavuah, a regular menstrual cycle, she becomes a niddah only at the moment that she sees blood and not before that point, retroactively, which would cause ritual impurity to have been imparted on anything she touched.  The rabbis acknowledge that most women cannot predict the exact day that they will menstruate.  However, a veset kavuah is established once she menstruates on the same day of the Hebrew month for three consecutive months.  

Rav Huna teaches us that women can create a regular cycle.  One example involves jumping - if a woman can perform a physical action that leads her to menstruate three months in a row, she has created a predictable cycle.  Rashi suggests that other actions like carrying something heavy or suffering from an illness might create a regular cycle as well.

The Ritva, Rashi and other rishonim suggest that when a woman may create a veset, a predictable cycle, she has to expect her period and become a niddah every time that she does that action.   The Rashba, Tosafot and others teach that a woman must have another factor involved to create a veset for a physical action.  For example, if she jumps three times on a specific day of the week and she menstruates each time, the women will establish a veset for jumping on that day.  If she jumps on different days she need not expect menstruation.

The rabbis seem to be balancing a number of factors: the need to understand menstruation, the need to understand women's control over menstruation, the need to connect their system of ritual purity with women's menstrual cycles, among other items.