Gemara

11a (Mishnah): A woman (who deals with Taharos) must check herself morning and evening, and before Bi'ah.

68b (Mishnah - R. Eliezer): If a (Zav or) Zavah checked herself on the first and seventh days that she must count, and found herself to be clean, and did not check on the other days, we rely on Chazakah that also the intermediate days were clean;

R. Yehoshua says, only the two days checked count. Five more clean days are required;

(Beraisa - R. Eliezer (to R. Yehoshua)): If you are concerned for Tum'ah in the middle, day one should not count! "V'Achar Tithar" obligates immersing after all the (seven clean) days (consecutively, without Tum'ah in the middle).

Answer (Beraisa): If a woman says "I saw blood one day" (she does not know when), she immerses seven times (the next seven nights) due to Nidah, and two times during the day due to Zavah. The Tana holds that it is a Mitzvah to immerse at the first opportunity.)

If she says "I did not see," she immerses 15 times.

Objection (Rava): It is absurd to immerse more often when she did not see!

Correction: Rather, if she does not know how she saw, nor whether it was during the days of Nidah or of Zivah," she immerses 15 times:

This shows that Chachamim checking each day.

Rejection (Rav Acha brei d'Rav Yosef): We corrected the Beraisa. If we amend it differently, the proof goes away! I.e. she says "I do not know whether or not I counted, or how many days I counted..."

Rishonim

Rosh (10:5): Nowadays that women are Safek Zavos, they must be Mafsik b'Taharah to check themselves, to know that the blood stopped. She must check herself each of the seven days. The custom is that women wear a white undergarment Bein ha'Shemashos beginning the seven clean days. She checks herself and her garment every day.

Tosfos (Nidah 15a DH u'Mar): According to the opinion that Vestos mid'Rabanan, she is Toleh on a wound even at the time of the Veses, i.e. if she feels that the blood comes from the wound. If Vestos were mid'Oraisa, we would be concerned lest blood came also from the Makor. If she does not feel blood coming from the wound, all are Metamei at the time of the Veses. Will she never be Nidah?! She had a Veses before the wound!

Chasam Sofer: Tosfos is Metamei at the time of the Veses, for we cannot say that she will never be Nidah. I.e., granted, she has Chezkas Taharah, and the Chazakah that the Veses comes in its time is not an absolute Chazakah, for Vestos mid'Rabanan. Even so, most women see Dam Nidah at least at some time. If we would always attribute her blood to the wound, we would say that she is not from the majority. A majority overrides a Chazakah. To say that she is always Temei'ah unlike most women who see blood for a limited time, and cease. Also, Chezkas Taharah helps her, and there is a wound in front of us. Therefore, we pick one day on which her Chezkas Taharah is weak. Even though Chezkas Veses is weak, for it is mid'Rabanan, also her Chezkas Taharah is weak. This is why she must refrain shortly before the Veses, lest she see. This is like Tosfos. The Rashba and Ran disagree. When one has a fresh wound that emits blood, all excess fluids from the body exude from there. Therefore, if she has a wound in the Makor, all excess fluids that normally come out with Dam Nidah at the time of the Veses, will exude from the wound. The blood of her Veses totally stops. This is not unlike most women, for perhaps this happens to most women with a wound there. Therefore, we permanently leave her on her Chazakah.

Poskim

Shulchan Aruch (YD 196:4): L'Chatchilah, she must check twice on each of the seven days, once in the morning and once close to Bein ha'Shemashos.

Beis Yosef (DH uv'Chol): The Ramban, Rashba, Rosh, and Hagahos Maimoniyos citing Tosfos wrote that l'Chatchilah, she must check every day. This connotes that once each day suffices. Sefer ha'Mitzvos says that she must check twice every day, and so say Semag, Semak, Sefer ha'Terumah, and the Mordechai in the name of the Roke'ach. She checks when rising in the morning, and in the evening. I say that they learned from 11a.

R. Akiva Eiger: L'Chatchilah she must check herself and her garment.

Pischei Teshuvah (4): The Shlah (p. 101) obligates her to count every day and say 'this is such and such day', for it says "v'Safrah Lah." Noda bi'Yehudah (2 YD 123) disagrees. Maharam bar Baruch (292) wrote that she does not count, for this is unlike Sefiras ha'Omer. Also the Radvaz (4:27) says that she need not count with her lips.

Chavas Da'as (3): The Gemara connotes that if she did not check, even if she was not Margish, she is Temei'ah mid'Oraisa, for we require counted and checked days. We established the Beraisa (of one who saw blood one day) when she counted, but does not know how many days she counted. Rashi explains that she checked. This implies that if she did not check, she need not immerse, even though the Tana holds that Tevilah bi'Zmanah Mitzvah. If mid'Oraisa the seven clean days do not require Bedikah, she should need to immerse! Also, the Poskim say that she must check cracks and holes, for there is Kares. This shows that it is mid'Oraisa. All the more so, blood without Hargashah (is Soser), for this does not require Bedikah.

Chasam Sofer (YD 177): Indeed, for a Hefsek Taharah, a proper Bedikah is required. This requires no proof. However, he brought no proof for the seven days. Surely, it does not help that all of Bein ha'Shemashos she did not feel her Makor opening. At the time of her Veses the Makor opened to let out blood. Perhaps it is still open and exuding blood, until she was Mafsik in front of us. Even if she did not feel it open, she must know that it closed! To know this she must check holes and cracks. However, after she did so, to count clean days, perhaps it suffices if one morning she was mindful and is sure that her Makor did not open. We must know that she did not feel, but forgot about it. This is very hard. Therefore, the primary Bedikah is through inserting a cloth into holes and cracks. If she does not find blood, she avoids all doubts. There is no difficulty from the Gemara. "I do not know how many days I counted..." means that she was not mindful of herself, and she did not check holes and cracks. Rashi and all the Poskim that the Beis Yosef brought explained the simpler Bedikah of holes and cracks. If a woman was not so mindful, and relied on Bedikah of a cloth, if she did not check holes and cracks it is a Safek Kares.

Chavas Da'as: Also, blood without Hargashah is Tamei for Taharos, for the place of the Makor is Tamei. Tum'ah interrupted, so "v'Achar Tithar" does not apply. Also, they are not clean and checked. Also, (semen) inside her is like outside, so it is considered seeing (blood - 42a). This applies also to blood without Hargashah. R. Akiva taught about a Kesem (58a) 'the Torah said "blood", and not a Kesem.' Why didn't he learn from "bi'Vsarah', that she must feel it inside herself? Shmuel uses this to be Metaher even actual blood without Hargashah (57b)! Rather, "blood" teaches that she cannot count a day to be clean if she had blood without Hargashah. We do not say so about a Kesem. The Torah is not concerned for Kesamim at all.

Chasam Sofer: He holds that blood without Hargashah is Soser, like semen. We hold that (mid'Rabanan) even a Kesem is Soser. Avnei Milu'im proved from the Ramban and Ran that mid'Oraisa, she is not Temei'ah due to the place of the Makor, until the blood leaves her. She is like one who touched, and not like one who saw. Therefore, mid'Oraisa it is not Soser at all.

Chavas Da'as (Bi'urim 3): If a woman saw real blood, and then developed a wound that emits blood, she will not be able to check herself even once without blood. Since she is Muchzekes Temei'ah, she cannot become Tehorah until she has a Hefsek Taharah and one day amidst the seven without any blood. However, if she checked one morning during the seven days and found herself Tehorah, we consider all the days to be checked and she is Tehorah. We attribute all blood afterwards to the wound. She needs a proper Hefsek Taharah with a cloth. Likewise, if a woman has a Veses (or if she does not, so day 30 is considered her Veses) and she has a wound that emits blood, she may not be Toleh on it. To leave Tum'ah she needs at least two clean Bedikos, one for a Hefsek Taharah, and one during the seven days. The Beis Yosef says that without Bedikah, there is Kares. The Gemara says that she is Toleh on a wound, i.e. when she first sees. Then, without checking she is Tehorah. It is as if she did not see. If she was Temei'ah, even if she would be Toleh on the wound, how can she leave Tum'ah without Bedikah for a Hefsek Taharah and the seven days?! Why did the Mechaber and Rema (187:5, brought below) say Stam (that she is Toleh on a wound) without saying that needs at least two clean Bedikos? Granted, we can be lenient about Bedikos during the seven days, due to a Sefek-Sefeka. Perhaps she did not see at all, for Vestos mid'Rabanan, and even if she saw, perhaps it was during Nidah, and not during Zivah, so (mid'Oraisa) she does not need seven clean days. What is a Safek to the entire world is not called a Safek due to lack of knowledge (Taz 99:6). However, even a Nidah requires Bedikah and a Hefsek Taharah, so there is no Sefek-Sefeka. Also for the seven days it is hard to rely on the Sefek-Sefeka. Perhaps 'perhaps she did not see' is not called a Safek, for surely eventually she will see! The Safek 'perhaps she saw during Nidah' is a Torah Safek. It seems that the Mechaber and Rema relied on what they wrote here, that she does not leave Tum'ah without two Bedikos. They should have elaborated.

Pischei Teshuvah (5, citing Chasam Sofer 177): If she was Tehorah, just we are concerned lest she saw at the time of the Veses, if she waited longer than the blood normally flows (during her period), presumably it stopped flowing. If she is mindful an entire Bein ha'Shemashos and not feel her Makor open, she can be Mafsik b'Taharah.

Shulchan Aruch (187:5): If she has a wound in that place, we are Toleh blood on the wound.

Rema: This is if she has a Veses Kavu'a. Not at the time of the Veses, she can be Toleh on the wound, even if she is not sure that it emits blood. Similarly, if she does not have a Veses Kavu'a, and it is a Safek whether the blood came from the Makor or from the sides, she is Toleh on the wound due to a Sefek-Sefeka. Perhaps the blood is from the sides. Even if it is from the Makor, perhaps it is from the wound. At the time of the Veses, or after 30 days if she has no Veses, she is not Toleh on the wound. If not, she would never become Temei'ah.