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1. The Gemara discusses whether blood that has the Halachic status of Tahor and blood that has the status of Tamei come from the same source in the body or from different sources.
2. The Gemara rules in accordance with Rav, that both Tahor and Tamei blood come from the same source.
3. The Mishnah discusses when a woman is considered a Yoledes b'Zov.
4. The Mishnah discusses how much time a woman may bleed before childbirth without being considered Yoledes b'Zov.
5. Blood due to childbirth is not deemed Tamei.
A BIT MORE
1. Rav: All blood, whether menstrual blood (which is Tamei) or blood that flows after seven days of birth of a boy (which is Tahor), comes from the same place. The Torah simply decrees when this blood is Tahor and when it is Tamei. Levi: Tahor blood comes from one source, and Tamei blood comes from a different source.
2. While Mar Zutra argues that one should be stringent and follow both Rav's opinion and Levi's opinion (see #1), Rav Ashi, Mereimar, and the Gemara conclude that we follow Rav in all situations, whether it results in a stringency or a leniency.
3. Rebbi Eliezer: If a woman bled for three days, stopped feeling pain of childbirth for 24 hours (but kept bleeding), and then gave birth, she is considered a Yoledes b'Zov. Rebbi Yehoshua: She must stop feeling pain for a full night and day (as opposed to a consecutive 24-hour period from the middle of one day to the next) before she is considered a Yoledes b'Zov.
4. Rebbi Meir: Even if there are 40 or 50 days of bleeding due to childbirth, she is not considered a Yoledes b'Zov. Rebbi Yehudah: Any bleeding within one month before the birth is due to childbirth. Rebbi Shimon: Any bleeding within two weeks before the birth is due to childbirth.
5. The Gemara states "her blood," implying blood that comes from her menstrual cycle as opposed to blood due to childbirth.
Index to Revach for Maseches Kerisus