1,078. Outside of the Normal Time

155:3 If a woman does not have a regular menstrual pattern, then the thirtieth day after the start of her last period is always like a set interim for her. This is called "onah beinonis" - an average time span. For example, if she saw blood on the Monday of the week in which parshas Noach is read, then the Wednesday of the week in which Toldos is read (30 days later) will be like a fixed interim for her. Aside from this, she must watch for any signs that her period is coming, whether they be fixed intervals or a given day of the month, so that the matter can be clarified. Her husband must separate from her 12 hours before the time when she must suspect that her period may be imminent.

155:4 Every woman who saw blood must suspect that she will do so again on another occasion on that day and at that time. This is obvious in the case of a woman who does not have a regular menstrual pattern. Rather, even a woman who has a regular menstrual pattern, if her period came unexpectedly and she saw blood at a time that wasn't the normal time for her period, she must watch for these signs in the future to see if her menstrual pattern has changed. If it happened just the one time and her period has not changed, then she need not be concerned about it. Since she has a regular pattern, she need not worry about an irregular pattern.