שבועות יח, ב
ת"ר "והזרתם את בני ישראל מטומאתם" אמר רבי יאשיה מיכן אזהרה לבני ישראל שיפרשו מנשותיהן סמוך לוסתן וכמה אמר רבה עונה
Our Rabbis taught: "You shall separate the children of Israel from their uncleanness"[Lev. 15:31]; R.
Yoshiah said: From this we deduce a warning to the children of Israel that they should separate from
their wives near their periods. And how long before? Rabbah said: One ‘onah [either the whole day or the whole night].
In tomorrow's page of Talmud, Rabbah (~270-330 CE), who lived in Babylonia, ruled that a couple must refrain from intercourse if menstruation is expected to begin within a twelve hour window. This ruling is included in the Shulchan Aruch, the code of Jewish Law, as the required Jewish practice:
שולחן ערוך יורה דעה קפד, ב
בִּשְׁעַת וִסְתָּהּ, צָרִיךְ לִפְרֹשׁ מִמֶּנָּהּ עוֹנָה אַחַת, וְלֹא מִשְּׁאָר קְרִיבוּת אֶלָּא מִתַּשְׁמִישׁ . אִם הוּא בַּיּוֹם, פּוֹרֵשׁ מִמֶּנָּהּ אוֹתוֹ הַיּוֹם כֻּלּוֹ אֲפִלּוּ אִם הַוֶּסֶת בְּסוֹפוֹ, וּמֻתָּר מִיָּד בַּלַּיְלָה שֶׁלְּאַחֲרָיו, וְכֵן אִם הוּא בִּתְחִלָּתוֹ, פּוֹרֵשׁ כָּל הַיּוֹם וּמֻתָּר כָּל הַלַּיְלָה שֶׁלְּפָנָיו
During her veset, [the expected onset of menstruation, her husband] must separate from her for one onah, not from all contact but only from marital relations. If her period is expected in the daytime, separate from her for that entire day, even if the veset is at the end of the day, and it is permitted [to have marital relations] immediately the following evening. Similarly, if [the veset] is at the beginning, separate the whole day and it is permitted the entire preceding evening...
This ruling suggests that women can predict when the onset of menses will be. How often is that in fact the case? Well, for the half of you who are men, it might surprise you to learn that this ability to predict the onset is less common than you would think. Women, I am sure, already know this.
How Regular is Regular?
In a 2011 review paper "The normal menstrual cycle in women," the authors point out that the 28 day"text-book" length of the menstrual cycle in young healthy women is in fact highly variable. Even between similarly aged women the cycle may range from 25 to 34 days. More to our topic, there are many women in whom the cycle length changes. In those aged around twenty, about 47% of women have a variation by as much as 14 days annually.
Another study on the variability of menstrual length from one cycle to the next comes from the Obstetrics and Gynecology Department at the University of Pittsburgh. They asked 130 women to keep menstrual diaries and record their menstrual flow for at least four menstrual cycles and for as long 30 weeks.
They found that 46% of all subjects had a cycle range of 7 days or more, and 20% had a cycle range of 14 days or more. In other words, almost half of the women had a cycle-to-cycle change of at least a week, and one in five had a change of two weeks or more. "Therefore" they wrote, "one out of every five subjects who reported that they had regular cycles were experiencing periods that occurred 1 week away from the expected date."
Irregularity in Jewish Law
For those who wish to learn more, there is a long section in the Shulchan Aruch (יורה דעה הל׳ נידה קפט) that addresses the complicated issue of irregular menses and the required time for a husband and wife to refrain from intercourse. This long section is needed because for many (?most) women, the length of the menstrual cycle changes from one period to the next. R. Yoshia's ruling that a couple must refrain from marital relations "סמוך לוסתן" - at the expected time of menstruation - turns out to be a rather complicated thing to do.