1,082. Those Who Are Not Expecting a Period
155:11 Just as a woman must suspect a menstrual pattern based on a given day of the month or after a given interval of days because of a single occurrence, similarly she must suspect a pattern based on physical symptoms or a combination of factors because of a single occurrence. And, just as patterns based on the day of the month and the interval of days before they are established can be broken by a single exception, patterns based on symptoms or a combination of factors are likewise broken by a single exception. Just as patterns based on the day of the month and the interval of days once established are only broken based on three exceptions, similarly, patterns based on symptoms or a combination of factors once established are only broken by three exceptions.
That is, if she had a physical symptom pattern only, if she later had that symptom on three occasions and didn't get her period, she need no longer consider that symptom a sign. If she had a combination pattern and on three occasions the day arrived with the symptom but she did not get her period, then the pattern is broken. However, the day without the symptom or the symptom without the day do not break a combination pattern because breaking a pattern requires the same combination of factors as establishing it.
155:12 A pregnant woman after the first trimester and a nursing mother are both presumed not to menstruate and they need not anticipate receiving their periods even if they had regular patterns. If they do menstruate, they must anticipate the possibility of doing so again, the same way that a woman with an irregular pattern does. When she finishes nursing, she returns to anticipating her original pattern. For example, if she used to start her period on Rosh Chodesh, she needs to check on the first Rosh Chodesh. If her period started after a given interval of days, she cannot anticipate it until she sees blood for the first time, after which she anticipates the interval that she had been accustomed to seeing.