We learned earlier how Shammai established no precautions at all, Hillel established too much, and the Sages took an intermediate position. What were their reasons?
Shammai said that if a woman sees blood, there is absolutely no ritual impurity applied to her retroactively, and that only ritually pure foods that she touches from now on will become impure. The Talmud found four possible reasons for his opinion. In the fourth approach, Shammai told to Hillel that if he declares a woman ritually impure retroactively, then he will also affect propagation and increase, because the pair will always be afraid that any blood she sees later will make her a niddah retroactively, and they will have transgressed.
Hillel answered that he is only talking about ritual food, something that would not apply outside the Temple or related requirements, but Shammai answered that even so they will be nervous.
The Sages felt that one position was too lenient, while the other too strict. Among their reasons was the prior Sages have established for women working with ritually pure foods a requirement to check themselves for blood every morning and evening. Since this requirement was taken seriously, they could rely on it and declare retroactive impurity only back to the previous check. If she missed a few checks, they still would not extend the impurity back more than twenty four hours, since it is inconceivable that a discharge would not be noticed for longer than that.
Art: Pierre Auguste Renoir - The Thinker Aka Seated Young Woman