1,081. Checking at the Expected Time

155:9 If a woman has a regular menstrual pattern, before the time arrives when she normally sees blood, she should insert a cloth in order to know for sure. This is because there is reason to be concerned that a drop of blood may come out and become lost, with the result that she doesn't know about it. If she did not do this and the expected time for her period passed, she may not have relations with her husband until she checks herself carefully. If during this time she washed herself so that checking won't help, then she should act stringently and consider herself ritually unclean based on the presumption that her period came at its regular time.

On the other hand, a woman who does not have a regular menstrual pattern and merely anticipates an unestablished time, if that time passed and she didn't feel anything, she is presumed to be ritually clean, even if she didn't check herself. However, the "onah beinonis," which is the thirtieth day (see 155:3), is considered like a regular menstrual pattern

155:10 A woman who sees blood for two or three days, whether she has a heavy or a light flow, the day she first starts seeing blood is the day that counts as the day her period is expected. Some authorities say that she must anticipate it on all these days until they have passed without her seeing blood.