1,080. No Established Pattern

155:7 If a woman doesn't have an established pattern, her husband should record the day that she saw the signs of her period - both the day of the month and how many days it was from the previous time. The wife must anticipate her period based on the interval between her last two periods as well as the day of the month of her previous period and the "onah beinonis" (the average span of time). This is how they should behave until they can establish a fixed pattern. After this pattern is fixed, if her period happens to change, they must also anticipate this new interval and the new day of the month in addition to her established pattern because the established pattern is not overturned until it has been violated three times, as we will see in the next halacha.

155:8 If a woman has a regular period but once or twice the expected day arrived or she experienced the accompanying symptoms and she didn't see blood - even if she is definite that she didn't see it, such as if she had a cloth inserted vaginally the entire time she was expecting her period - nevertheless, her menstrual pattern is not disregarded and she must still check the third time. If the third time arrives and she still hasn't seen blood, if she's completely sure that she didn't see blood all three times - such as if each time she had a cloth inserted vaginally the whole time she was expecting to - then her existing pattern is disregarded and she no longer needs to anticipate her period based upon it. This is because a pattern is discarded if it is violated three times, even if it was reliable for a number of years.   However, when it comes to a pattern that was not established - meaning that it only happened once or twice - if the proper time or symptoms arrived and she did not get her period even once, even though she didn't check herself and merely didn't feel or see anything, then she no longer need anticipate this time. This is because whatever is not established by happening three times in succession is cancelled after just one exception, even without checking.