1,079. Anticipating a Period

155:5 Let us take as an example the case of a woman who has not yet established a menstrual pattern. If she sees blood on the second day of Rosh Chodesh Iyar and again on the twenty-fifth day of that month, she must watch for Rosh Chodesh Sivan because maybe her period will regularly fall on Rosh Chodesh. She must also watch the second day of Sivan because of the "onah beinonis" (the average span of time). If she doesn't see blood on Rosh Chodesh Sivan or on 2 Sivan, then she must watch to see if her menstrual pattern will follow set intervals. Since the time between seeing blood is 23 days - not including the actual days on which she saw blood - she must now count 23 days and then anticipate her period. This leaves four days to the month of Iyar; add the first 19 days of Sivan (to make the anticipated 23-day interval) and she should anticipate her period on 20 Sivan. If 20 Sivan arrives and she still has not gotten her period, she must anticipate 25 Sivan, in case her menstrual pattern is the twenty-fifth day of each month. She must also anticipate her period on 26 Sivan in case she has an "onah beinonis." She need not anticipate her period on Rosh Chodesh Tammuz because the hypothesis that her period would start every Rosh Chodesh was already disproved when she didn't see any blood on Rosh Chodesh Sivan; any scenario that does not occur three times in succession is undone by one that doesn't fit the pattern.

155:6 Let us also take the case of a woman who had already determined a menstrual pattern in which she gets her period every twenty-five days. If her period changed once and she didn't see blood until the twenty-eighth day, she must now check on the twenty-fifth day to see if this is still her time. If the twenty-fifth day arrives and she hasn't gotten her period, she must anticipate it on the twenty-eighth day because of the previous incident. If she sees blood on the twenty-eighth day, she is still not allowed to engage in marital relations on the twenty-fifth day because of her previously-established menstrual pattern. If again she doesn't see blood on the twenty-fifth day but she does on the twenty-eighth day, then her menstrual pattern is set at 28 days. The previous pattern of 25 days is eliminated because it was not upheld for three consecutive times. However, if her new intervals were not consistent, such as if she saw blood on the twenty-eighth day, then on the twenty-ninth day and then on the thirty-first day, then her original menstrual pattern is overturned without establishing a new pattern. She must then always anticipate her period based on the most-recent interval of days, the day of the month, and the "onah beinonis" until a new pattern is established.