A woman who excreted a drop of blood from her womb to her vagina is considered impure; her touch on food and vessels impurifies them. Shamaai and Hillel disagreed about a woman who was pure, examined her vagina with a piece of cloth, and found a drop of blood on the cloth. From what point is she impure? According to Shamaai the woman is impure from that moment forth and her touch impurifies food and vessels from that moment forth. According to Hillel the woman is impure retroactively back to when she last examined herself and was clean of blood. The scholars explain the disagreement by stating that Shamaai thinks we do not suppose the drop of blood remained in her vagina for days before its discovery, for the drop would have fallen out. Since it did not fall out, only now did it leave the womb for the vagina. According to Hillel one must fear the drop left womb days before and did not fall out because the vaginal walls kept it from falling out. According to Shammai, the vaginal walls cannot keep a drop of blood from falling out. The scholars asked: According to Shamaai, a woman who does not wish to become pregnant and customarily places a piece of cloth in her vagina to absorb the sperm — is she impure retroactively or only from the discovery on? According to the scholar Abaye, even Shamaai would agree that she is retroactively impure, for the cloth embedded in her vagina would not allow the blood to seep out, so there is a fear that the drop of blood has been in the vagina for many days. According to the scholar Rava, even in the case of a woman who keeps a cloth in her vagina Shamaai would think she was impure only from the discovery onward, because there is a high likelihood that because of the woman’s sweat the cloth will contract in her vagina and the blood would have place to fall out. Since it did not fall out, it only now left the womb. Rava added that if the cloth is compressed and pushed tightly against the vaginal walls so that it had no room to contract and give room for the blood to flow out, Shamaai would admit that she is retroactively impure.
(Babylonian Talmud, Tractate Niddah 2a-3a)