It is the custom of the daughters of Israel when having marital intercourse to use two testing-rags, one for the man and the other for herself,and virtuous women prepare also a third rag whereby to make themselves fit for marital duty.
The question of performing an examination prior to marital relations was discussed above (see daf 12). The main discussion in this Mishna, however, is the need for an examination after relations have taken place. Rashi and the Ramban explain that the need for such an examination stems from the possibility that intercourse itself caused the woman to menstruate, so both she and her husband will need to make certain that no blood was found.
The Gemara is unclear to whom this Mishna refers. According to Rashi this custom was instituted only for women who were going to be in contact with consecrated food. Such women ordinarily examined themselves every morning and evening, and this was an expression of additional sensitivity to this holiness. Other women never practiced this custom.
Rabbenu Hananel rejects this explanation and applies this custom to all women who do not have a set menstrual cycle, explaining that we are concerned that such women may menstruate because of outside influences, such as intercourse. Since the intercourse may have caused her to become a niddah she is forbidden to her husband, unless she ascertains that she is ritually clean.
The Rambam understands that the custom mentioned in the Mishnah applies to all women under all circumstances (see Mishneh Torah Hilkhot Isurei Bi’ah 4:14). The Rema rejects this stringency, however, and says that we do not accept this as the halakhah (see Shulhan Aruk Yoreh De’ah 186:1).