2,105. Retroactive Impurity and Its Exceptions
Hilchos Issurei Biah 9:3
Also by rabbinic enactment, if a woman sees blood not at her designated time or if she sees a blood stain, she is retroactively ritually unclean for 24 hours; if she conducted a self-inspection within the preceding 24 hours and found that she was ritually clean, she is only ritually unclean retroactive to the time of her self-exam. Even though she is unclean retroactively, she doesn’t render unclean a man who had been intimate with her, as was previously explained. Similarly, she doesn’t begin counting her niddah days or counting because of the stain retroactively; she only does so as of the time she saw the blood or the stain. Whenever a woman finds a stain, the calculation of her designated time is affected because of the possibility that the blood may have come from her uterus; if this is the case, it would require recalculating her designated time.
Hilchos Issurei Biah 9:4
If a woman sees blood at her designated time, she is not rendered ritually unclean retroactively, but rather as of the time she found the spot. Similarly, a pregnant or nursing woman, a virgin and an elderly woman are not rendered ritually unclean retroactively. A pregnant woman means one who is visibly pregnant, i.e., three months pregnant. A nursing woman means one who is within 24 months of giving birth, even if the child died, was weaned or given to a nursemaid.