Niddah 68 - When to Examine Oneself
A woman who became a niddah may purify herself seven days after her initial discharge, is she knows with certainty that the flow of blood has ceased prior to the conclusion of the seventh day. (That, of course, is the Torah law, but we mentioned that the modern law is more uniform, though more stingent). When does she need to perform this conclusive examination?
If she examined herself in the morning of the seventh day, and found herself to be pure from blood, then she is presumed to be so later. Even if she did not perform an additional examination at twilight, and after several days she examined herself and found blood, she is presumed to be pure in the interim, and the ritually pure foods that she handled after immersion are definitely pure. Rabbi Yehudah disagrees, and requires an examination right next to the end of the seventh day, because he suspects that the flow of blood may have resumed after morning examination. However, the Sages give a definite lenient ruling: even if she examined herself at the beginning of the second day, and found herself pure, she has the presumption of purity from then on.
Art: Frans van Mieris - Young woman in the morning