Menstruation is often a predictable phenomenon. Sometimes the woman has a physical sign which indicates to her that menstruation is imminent. Other woman are able to know when they will menstruate based on the interval from their last menstruation. The next chapters will discuss the halachic ramifications of woman who have predictable, fixed periods.
1. A woman who does not have a fixed period must separate from her husband (see chapter on separating when a period is expected) on the thirtieth day after each period. The day her menstruation begins is considered the first day of the thirty. This is because an average period is thirty days. This thirty day period is considered like a fixed period for a woman without a fixed period or regular time for menstruation.
2. Additionally, a woman who does not have a fixed period must separate from her
husband at other times because she might be in the process of fixing her period. These
times are: the time of the previous interval; and the day of the Hebrew month that the
last menstruation fell on. The previous interval is the number of days between the last
two menstruations. For example, if the woman menstruated today during the night and thirty
two days ago, either day or night she must separate from her husband in thirtytwo days at
night.
diagram #1
separate

32
today 32
3. There are many ways that periods are fixed. It is important for the woman to keep a record of the dates on which they menstruate in order to determine if there is a regular predictable pattern. The record should include the (Hebrew) date of the menstruation and whether it occurred in the daytime or the evening. One should also calculate the number of days between the onsets of menstruation and any physical signs preceding the menstruation (This will be discussed later).
4. There are basically two different kinds of fixed predictable periods: periods related to time and periods related to physical signs. There are also periods which rely both on time and physical signs.
5. The general rule is that three consecutive occurrences of the same pattern (date, interval or physical indicator) fixes the period. (Note to fix a period based on the interval it will take four menstruations.) This pattern is associated with time and/or a physical indicators.
6. A period of an equal interval of days is probably the most common: it is fixed with four menstruations occurring at three equal intervals between them. For example, if a woman sees every thirtytwo days at night, after she menstruates four times each time commencing 32 days from the previous time she is considered fixed to have an interval period of thirtytwo days and must separate from her husband every thirtysecond night for the whole evening (diagram #2). If she did not menstruate that evening she is permitted to her husband in the morning (As discussed in chapter XIV, 21). Even though she has not fixed an interval period four times she must separate after the first interval is seen i.e. she saw twice at a thirtysecond day interval she must separate (abstain from sexual intercourse) in anticipation of the third time.
diagram # 2 sep. sep. 1 2 3 4 from now on seperate every 32 > 32 32 32 32
7. In order to have a fixed period, all of the occurrences of menstruation must either be in the day or in the evening. If they are off by even a few minutes even though the interval is almost the same, she does not fix a period.
8. If the woman does not have a fixed period the couple must abstain from sexual
intercourse on the interval between the last two menstruations. Additionally, as discussed
above, in the absence of a fixed period the couple must abstain from sexual intercourse on
the day of the month that their last period was, and on the thirtieth day after the last
period. (see diagram # 3)
diagram #3
Tishre,
6 Heshvan, 6
XY

day 32, last interval
day
30
9. Menstruation which occurs on the same day of the Hebrew month or on the same day of the week is considered a fixed period once it has occurred three times. (see diagram #4) 9.
diagram #4a from here on her
periods are fixed to
occur on Rosh Hodesh
first of 1 of 1 of Tishre Heshvan Kislev > diagram #4b from here on she has preiods for every fourth Wednesday Wed. Wed. Wed. Wed. > 4 weeks 4 weeks 4 weeks
10. In addition to the normal interval period there is a period which changes following an organized pattern, this is called a skipping interval period. ie., she adds one day each time: first she menstruates at an interval of thirty then thirtyone then thirtytwo then thirty three, since she has menstruated four times she acquires all of the strictures of having a fixed period. (see diagram #5) She must follow it until it has been uprooted three times as will be later discussed. 10.
diagram #5
Today
30 days 31 days 32 days
33 days
>
11. Such a skipping interval period can add several days or even subtract days, ie.,
she menstruated today and again inthirty days then twentynine then twentyeight
then twentyseven, she has fixed as skipping period which subtracts.
12. The couple need not abstain due to a skipping period until it is fixed, instead they separate at all other times for the previous interval, the thirtieth day, and the corresponding day of the Hebrew month.
13. Even if a woman menstruates in a regular fashion she has not fixed any period unless it is always during the same time period: or day or night.
14. If a woman menstruated three times during the daytime she has fixed a period which occurs during the day. If she then menstruated a fourth time at night she must separate both day and night. Night because of a new period she might be fixing and day for her previously fixed period. If her period is not fixed she only separates for the last one that if she menstruated at night she only separates then, and similarly if she last menstruated during the day she separates from her husband during the day.
15. Considering the complications of these rules it is important for women to keep an accurate record of her periods and if need be should ask her rabbi to figure out if she has a fixed period.
16. Just as it takes three times to fixed a period, it takes three times to uproot a fixed period. That is to say if her period changed once or twice she still separates for her old fixed period. If she changed a third time and her period are run in a different order she has uprooted the old one and fixed a new one.
17. A woman changes from one period to another in the following way: If she had a set period say for every thirtyone days and has now changed to twentyeight the first time it happens she must separate on the twentyeighth. If she did not then see, three days later she separates for the regular thirtyone day period. It is the same for the second time: she next separates on the twentyeighth and if she did not see a third time on the twentyeighth she worries for her old regular period on the thirtyfirst day. Upon seeing the twentyeighth day for the third time she has fixed a new period and her old thirtyone day period no longer exists.
18. If a woman had a fixed period and changed three times in a irregular fashion her old period has now been uprooted and she is now considered not to have any fixed period. It is the same rule if she completely ceased having her period three times. However, if she went back to her old fixed period even once it goes back to being fixed, since she has not fixed any new period.
19. Uprooting a fixed period, associated with the day of the month is the same in that she must change three more times from her old period until it is uprooted by not menstruating three times on the expected day. 20. A woman who does not have a fixed period for a specific time but knows that at particular times she will definitely not have a period, i.e. she never menstruates at intervals of less than thirtytwo days or she never menstruates on intervals greater than twentyeight days, such women only need to separate at the time of the previous interval and not at the day of the month or the thirtieth day. This is called a half fixed period, if she changed only once it is uprooted like a nonfixed period but is strong enough for her not to expect menstruation on the day of the month or the thirtieth day.
21. This rule only applies for women who always menstruates in an interval of less than twentyeight days or in an interval of more than thirtytwo days but if she sometimes menstruates in an interval of between twentyeight and thirtytwo days she must still separate for the day of the month and the thirtieth day in addition to the previous interval.