2,135. Later Enactments

Hilchos Issurei Biah 11:3

In the Talmudic era, many doubts arose as to the appearance of blood and the calculation of menstrual patterns as not all women were able to figure their niddah days and zivah days. The Sages there acted stringently in this matter and enacted that women should consider all days as zivah days and consider any blood found as zivah blood out of doubt.

Hilchos Issurei Biah 11:4

The Jewish women accepted an additional stringency upon themselves: wherever Jews live, the practice is to observe seven clean days whenever a woman sees blood even if it’s no larger than the size of a mustard seed and the bleeding stops immediately. This is the case even if she saw the blood during her niddah days. It makes no difference whether she saw blood for one day, two, a full seven or longer: when the bleeding stops, she counts seven clean days like a major zavah and immerses on the night of the eighth day even though there’s a doubt as to whether or not she’s a zavah. In extenuating circumstances, she may immerse by day on the eighth day, as was explained (in halacha 4:8). After this, a woman is permitted to her husband.