2,076. Retroactive Impurity

Hilchos Issurei Biah 6:14

By “tenuous” (in the previous halacha) we mean that if the day on which she immersed goes by without the woman seeing blood, then everything she touched after immersion is ritually clean and one is not liable for being intimate with her. However, if she sees blood on the day after immersion, she is a zavah retroactively; anything she touched is ritually unclean retroactively, and both she and a man with whom she was intimate must bring an offering. Accordingly, such a woman is prohibited to her husband until evening in order to avoid a situation of doubt.

Hilchos Issurei Biah 6:15

If a zavah counts six clean days and sees blood on the seventh day – even close to sunset – then all of the days she counted are voided and she must begin counting seven clean days again after the unclean day.