Gemara

(Beraisa - R. Eliezer): If a woman miscarried a piece of flesh, even if there is blood inside, she is Teme'ah only if blood came with it. "Bi'Vsarah" excludes in a Shefir (the outer skin of a fetus before bones form) or piece of flesh.

Chachamim say, this is not Dam Nidah, rather, blood of the piece of flesh.

They argue about a convoluted piece. (The blood is between the folds);

R. Eliezer holds that "bi'Vsarah" excludes in a Shefir or piece, and likewise in a tube, but only if the piece is smooth. Blood between the folds of a convoluted piece is called "bi'Vsarah";

Chachamim are Metaher even then. The blood is of the piece. It is not Dam Nidah.

Inference: If the blood were Dam Nidah, she would be Teme'ah, even though there is a Chatzitzah. The same applies if it left in a tube!

Rejection (Rava): No. All agree that it is not normal for blood to leave this way, and that she is not Nidah. They argue about whether the blood itself is Tamei:

R. Eliezer holds that all blood from the Makor is Tamei;

Chachamim hold that since the blood does not make her Nidah, it itself is Tahor.

36a (Beraisa): If a Yoledes b'Zov (a Zavah who gave birth) counted her clean days but did not immerse, Beis Shamai and Beis Hillel argue as they do regarding Yoledes.

Levi explains that she gave birth to a girl. She counted seven clean days, and the blood started during the second week of Yemei Leidah (14 days of Tum'ah after birth of girl) and continued flowing into Yemei Tohar (days after Tum'as Yoledes). They hold that clean days of Yemei Leidah count towards the seven clean days.

67b - Question (Rav Papa): Nowadays, women are stringent to consider themselves Safek Zavos (like R. Zeira taught). They should be allowed to immerse on the seventh (clean) day! (Only a Nidah must wait until night, and the seven days of Nidah already passed.)

Answer (Abaye and Rava): R. Shimon's teaching forbids this;

(Beraisa): "V'Achar Tithar" -- after all the (seven clean) days. Tum'ah may not interrupt in the middle;

R. Shimon expounds "v'Achar Tithar" -- after an action, she is Tehorah (she may immerse in the morning of day seven). However, Chachamim forbid doing so, lest she come to a Safek. (If she will see later in the day, her Tevilah was invalid.)

57b (Shmuel): "Bi'Vsarah" teaches that she is Teme'ah only if she felt the blood in her body.

Question: "Bi'Vesarah" excludes blood in a Shefir or piece of flesh!

Answer: This is learned together with Shmuel's law.

Kerisus 8a (Beraisa): If a Zav had three sightings on the eighth night, he brings a Korban.

(Rav Huna bar Acha): The case is, he had only two previous sightings. Had he seen during the seventh day, he would be exempt from a Korban;

The Tana holds that a sighting that is Soser (interrupts a count of clean days) does not join to obligate a Korban.

Rishonim

Rif and Rosh (Shevu'os 4a and Nidah 4:20): Since a Yoledes b'Zov needs seven clean days, we apply R. Zeira's stringency to require seven clean days after any sighting of blood, even during Yemei Tohar.

Rosh: If the first week of a Yoledes Nekevah was clean, and she saw in the second week, this is not Soser her seven clean days, even though the blood is Tamei. A Beraisa (36a) teaches this. Blood in the second week is not Soser the seven clean days she counted. We can learn from here that if a regular Zavah counted seven clean days and did not immerse, and then she saw blood, she is not Soser. Even though the blood makes her a Nidah, she immerses from Nidah and the Tevilah counts also for her Zivah. One could distinguish, for blood during Yemei Leidah does not make her a Nidah or Zavah. However, presumably Stirah is only during the days she counts. We expound "Achar Tithar" - after all of them, without an interruption of Tum'ah. We are not concerned for Tum'ah after the count and before immersing. Nowadays, this question (about a regular Zavah) makes no difference, since for any blood she obligates seven clean days.

Rambam (Hilchos Isurei Bi'ah 6:15): If a Zavah counted six clean days, and on day seven she saw blood, even close to Shki'ah, this breaks the count and she returns to count seven clean days after the Tamei day.

Rambam (7:8): If a woman (in Yemei Tohar) counted seven clean days and did not immerse, and then she saw blood, she immerses and she is permitted to her husband immediately. Nidah and Zivah do not apply to the days of Tohar. However, the blood itself is Tamei and Metamei like blood of a Nidah, until she immerses.

Hagahos Maimoniyos (1): Also R. Tam and Sefer ha'Mitzvos (Lavim 11) rule like this. However, the Re'em says that is Soser, and he erased R. Tam's ruling from his edition of Sefer ha'Yashar. Also Avi ha'Ezri says so, and the Rif (Teshuvah 5 at the end of the Sefer) and the Roke'ach.

Note: I could not find the Teshuvah. Hagahos Maimoniyos (Constantina edition) derives from the Rif in Perek 2 of Shevu'os that it is Soser.

Or Same'ach: Tosfos (below) was unsure about one who counted seven clean days and saw blood on day eight before immersing. He concludes that presumably, she is not Soser. Kerisus 8a supports this. If a Zav saw three times on the seventh day, he would be exempt, for only Zivah that is Soser obligates a Korban. If a Zavah's sighting (after seven clean days) were Soser, surely a Zav's sighting after seven clean days and before Tevilah would be Soser. If so, even three sightings on day eight would not obligate a Korban. Why didn't the Gemara teach this?! Rather, even before Tevilah it is not Soser, and the same applies to a Zavah. The Shitah Mekubetzes wrote like this.

Shitah Mekubetzes (8a:9): Our Gemara proves that if a Yoledes saw blood after seven clean days before Tevilah, she is not Soser. Had she immersed, her blood would be Tahor. Even now that she did not immerse, Lo Sispor (she need not count; perhaps this should say 'Lo Sistor', i.e. she is not Soser - PF) and she immerses whenever she wants. Similarly, if a woman immersed, had Bi'ah, and then found that her Tevilah was invalid, her emissions of semen are not Soser. Perhaps there is no proof from here, for here we discuss after he immersed on the eighth night. However, there is a proof from Nidah (Perek 4).

Meshech Chachmah (Vayikra 15:28): "V'Achar Tithar" teaches that Yemei Leidah in which she does not see blood count towards seven clean days. She waits until the end of Yemei Leidah, and then immerses. This is like the opinion that "Achar" means 'long after.' According to the opinion that "Achar" means 'right after', it teaches that if she counted seven clean days during Yemei Tohar, and saw on day eight, it is is not Soser, since it is after counting. She immerses and she is permitted, even if blood is flowing.

Tosfos (36a DH Shavu'a): R. Tam learns from here that a Yoledes b'Zov, or nowadays any Yoledes, if she saw blood during Yemei Leidah after seven clean days, she is not Soser. Here we say that the blood she saw in the third week is not Soser, even though the blood is Tamei. All the more so, Dam Tohar is not Soser! One cannot learn from here that a Zavah (not a Yoledes) who saw blood after seven clean days is not Soser. Perhaps there is different, for the blood was Metamei her. Here, in any case she is Temei'ah for two weeks. Even though blood in the third week is Metamei her, it does not make her a Nidah or Zavah. Once the blood stops, she is permitted immediately. In any case, it seems that it is not Soser also for a regular woman. Presumably, Stirah is only within the count. "Achar Tithar" teaches that Tum'ah may not interrupt, but blood after counting does not interrupt. However, since we hold like R. Zeira, obviously it is Soser. Even if she immersed and saw blood, she needs seven clean days.

Or Same'ach: If she had blood without Hargashah, Acharonim argue about whether or not it is Soser. It seems that it is Soser, for Rava concluded that it is not normal to see blood in a piece. The Sar mi'Kutzi explains that it is Dam Nidah. They hold that the Torah is Metaher blood in a tube because it is not normal to see blood this way, or in a piece, even if it is very folded, for it is abnormal. It is not due to Chatzitzah. What forced Rava to say that they argue about whether or not the place of the Makor is Tamei? They did not mention this! He should say that R. Eliezer holds that even though "bi'Vsarah" excludes in a tube or piece, for it is not normal to see this way, still it is Soser the seven clean days. R. Eliezer holds that blood seen b'Ones does not cause Tum'ah or a Korban (37a), and even so it is Soser. Rabanan hold that what does not cause Tum'ah or a Korban is not Soser. Rather, Rava did not say so because all agree that it is Soser. "Bi'Vesarah" excludes blood in a Shefir, and also blood without Hargashah (57b). Since we proved that blood in a piece or tube is Soser, the same applies to blood without Hargashah, since one verse teaches both of them.

Poskim

Shulchan Aruch (YD 196:10): The seven clean days must be consecutive, without blood. If she saw even at the end of the seventh day, she is Soser all the days. She needs a Hefsek Taharah and must count seven clean days again.

Beis Yosef (DH u'Mah): The Tur says that if after the seven clean days she saw Bein ha'Shemashos, which is a Safek, she is Soser. This implies that if she saw at night, she would not be Soser, according to the Gemara (letter of the law). She would begin Nidah. However, after R. Zeira's stringency, in any case she needs seven clean days.

Taz (3): It is difficult to say that the Tur wrote something not relevant nowadays. Also the Beis Yosef wrote this in the Shulchan Aruch! I say that it pertains to Vestos. A woman with a Veses need not check at all, for she has Chezkas Taharah. During the seven clean days, l'Chatchilah she needs Bedikah, for she has Chezkas Tum'ah. The Tur teaches that this is until the very end of the seven days (even Bein ha'Shemashos), and therefore l'Chatchilah she must check herself then.

Rebuttal (Sidrei Taharah 32): The Tur needed to teach that even at the end of the seven days she is Soser all of them. This is why we forbid Bi'ah on the seventh day! If blood on day seven were not Soser, Bi'ah would be permitted, for even if she sees afterwards, that begins a new Nidah, and the Bi'ah was permitted. The Beis Yosef later retracted, and included this in the Shulchan Aruch.

Bach (11): The Tur discusses one who immersed on the seventh day, like the Torah permits. If she did not see Bein ha'Shemashos, presumably she had Bi'ah right after Bein ha'Shemashos. Since her count helped to permit the Bi'ah, her next blood begins Zavah according to R. Zeira's stringency. Had she seen Bein ha'Shemashos, we say that this is Soser her entire count, for it never permitted Bi'ah to her. What the Beis Yosef wrote is difficult.

Note: It seems to me that the Bach means that the Tur uses 'Soser' to mean that the count did not help, for it never permitted Bi'ah. Essentially, he teaches like Sidrei Taharah. Tif'eres l'Moshe explains like Sidrei Taharah, and concludes 'if the Bach meant like this, his words are very unclear.'

Imrei Baruch (in Yalkut Meforshim in Friedman Shulchan Aruch): The law of Stirah is relevant according to the opinion that does not rely on Sefek-Sefeka (two doubts) against a Chazakah. If there is a Sefek-Sefeka whether or not she saw blood, we cannot be lenient at a time when blood is Soser, for she had Chezkas Tum'ah. Alternatively, if Leah wore a garment during her clean days, and lent it to another woman, and Leah saw blood that was Soser, she had Chezkas Tum'ah. Therefore, if blood was found on the garment, we would attribute it to Leah.

Shach (12): The same applies if she saw blood on the eighth day. She always needs seven clean days before Tevilah. This is obvious.

Encyclopedia Talmudica (Zavah, footnote 222): How can he say that this is obvious? Rishonim argue about this! Perhaps he means that nowadays, the law is obvious, but his words do not connote like this.