As we learned in the first Mishna (see above, daf 2), Shammai and Hillel disagree about when a woman who menstruates is considered a nidda with regard to making things that she touches ritually unclean. Shammai rules that she only makes things unclean from the time she sees menstrual blood; Hillel rules that everything with which she came into contact since the last time that she examined herself for purity will be rendered unclean. The Sages disagree, arguing that Shammai is too lenient and that Hillel is too strict; they offer a compromise position.
That discussion, however, is limited only to women who do not have a regular menstrual cycle. The Mishna teaches that a woman who has a veset kavu’a – a set menstrual cycle – need not be concerned that she may have become a nidda before seeing blood; she can rely on the fact that the blood is the indication of her status as a nidda. The Mishna then goes on to discuss the status of a woman who examines herself for blood before and after sexual relations.
On today’s daf, the Gemara discusses the need to perform an examination before and after sexual relations, and appears to require both husband and wife to perform such an examination. The Ra’avad explains, however, that that requirement only existed in the time when the laws of ritual purity were kept, and since there were additional requirements established for the purpose of determining whether the people were ritually unclean for the purpose of coming into contact with consecrated objects, they were applied to the laws of sexual relations, as well. The Rambam argues that the requirement to perform such examinations applies even after the laws of ritual purity were abandoned, but that they are only a requirement for a woman who does not have a veset kavu’a.
The halakha in this case is that a woman who has a veset kavu’a need not – and, in fact, should not – examine herself for menstrual blood after relations. In the case of a woman who does not have a veset kavu’a, both husband and wife should examine themselves after the first three times that they engage in relations. If no blood is seen, then we can establish that the act of intercourse does not lead the woman to bleed and there is no need for examinations in the future.