Gemara

(Mishnah): A woman is Muchzekes to be Tehorah regarding her husband.

15a (Mishnah): If he returns from traveling, his wife is Muchzekes to be Tehorah.

(Rabah bar bar Chanah): This is even if he returned after her Veses came.

He holds that Vestos (when a woman normally becomes Nidah) are mid'Rabanan.

(R. Yochanan): If a woman has a Veses, her husband (when he returns home) can calculate when her last sighting should have been. If enough time elapsed since then for her to immerse, he may assume that she is Tehorah.

(R. Aba): He permits when it is possible that she never became Teme'ah, for then, even if she saw, perhaps she immersed.

16a (Rav Nachman): Rav Huna said in the name of Rav that if a woman did not check herself at the time of her Veses, and saw Dam later, she must be concerned for the time of her Veses, and for the time she saw. (These affect Taharos that she touched, and when the days of Nidah begin.)

Version #1: (She is concerned for the time of her Veses.) This shows that Vestos are mid'Oraisa.

Version #2: She is concerned for the time of her Veses only because she saw later. This shows that Vestos are mid'Rabanan. (end of Version #2)

(Rav): If a woman did not check herself at the time of her Veses, and checked herself later:

If she found herself to be Teme'ah, she is Teme'ah. If she found herself to be Tehorah, she is Tehorah;

(Shmuel): In either case she is Teme'ah. We assume that the blood came in its usual time.

(R. Zeira): All agree that Vestos are mid'Oraisa;

Rav discusses one who checked herself within Shi'ur Veses. Shmuel discusses one who checked herself after Shi'ur Veses.

(Rav Nachman bar Yitzchak): They argue about whether Vestos are mid'Oraisa.

(Rav Sheshes): Rav and Shmuel argue as the following Tana'im do:

(Beraisa - R. Eliezer): (If a woman did not check herself at the time of her Veses), she is (Muchzekes to be) a Nidah;

R. Yehoshua says, she checks herself later. (If she finds herself to be Tehorah, she is Tehorah.)

In another Beraisa, R. Meir holds like R. Eliezer, and Chachamim hold like R. Yehoshua.

Rishonim

Rif (Shevu'os 4b): If one returns from a journey, he may cohabit with his wife, whether she is awake or asleep, but only if he left her in Chezkas Taharah.

Ran (DH v'Garsinan): The Rif and Rambam hold that Chachamim (and similarly R. Yehoshua), who say that she must check, come to argue with the first Tana, who says that if she did not check, she is Temei'ah, for Vestos mid'Oraisa. They disagree, and say that she must check at the time of the Veses. I.e. this is the only stringency of Vestos. It is a stringency mid'Rabanan. If the Veses passed, why should she check? Any woman with a Veses, not at the time of the Veses she is like one who does not normally see blood. This is why she is Dayah Sha'atah. Why should we be concerned for blood when it is not common, just because she did not check at the time of the Veses? Rather, she must check at the time of the Veses. We say that if a woman with a Veses did not check at the time of the Veses, and checked afterwards, Rav holds that she is Teme'ah or Tehorah, based on how she found herself then. We infer that he holds that Vestos mid'Rabanan. This does not mean that she must check after the Veses. Rather, Rav discussed checking and finding herself Tehorah in order to argue with Shmuel, who says that even then she is Temei'ah. This is a good answer for the Rif and Rambam, but it is an excessive leniency not to be concerned for the Veses or Onah (average period of Stam women, i.e. 30 days) at all, even for many days, and even if she is sleeping.

Rambam (Hilchos Isurei Bi'ah 8:13): If a woman did not check herself at the time of her Veses, and checked herself later and found herself to be Teme'ah, even though regarding Taharos she is Teme'ah retroactively from the time of the Veses, she is not Metamei her husband retroactively, and she counts (the days of Nidah) only from when she saw. If she found herself to be Tehorah, she is b'Chezkas Tehorah.

Magid Mishneh: If her Veses passed and she was not Margish (have her usual sensation when the blood leaves the Makor), the Rambam forbids Bi'ah until she checks. This refers to a Veses Kavu'a. If it is not Kavu'a, since she was not Margish that day, she need not check. The Rashba says that since she does not have a Veses Kavu'a, she is concerned for day 30 from her (last) sighting, since this is the average Onah for Stam women. He holds that this is like a Veses Kavu'a regarding concern. If the day passed, she must check. Regarding a Veses that is not Kavu'a, he said that if it is less than the average Onah, e.g. she saw after 25 days, even if she did not check, since she was not Margish blood, she is Tehorah without any Bedikah at all.

Rosh (1:1): We do not hold like the Mishnah of a woman in hiding. Even though the Reisha says that if the time of the Veses came and she did not check she is Temei'ah, we say below (39a) that the entire Mishnah is R. Meir (who says that Vestos mid'Oraisa, but Chachamim disagree).

Mordechai (Shevu'os 735, citing Re'em): R. Zeira explains that Rav is Metaher if she checked herself within Shi'ur Veses, i.e. immediately, of her Veses. If not, not in any case she is Temei'ah. Even though there are Amora'im who disagree, we are concerned for R. Zeira.

Poskim

Shulchan Aruch (YD 184:9): Other women (who normally see blood) must check when the Veses comes. If the Veses passed and she did not check and was not Margish, she is Tehorah without checking.

Beis Yosef (DH v'Davka): The Rashba forbids other women until they check. R. Yehoshua and R. Eliezer argue about this. We know that the Halachah follows R. Yehoshua against R. Eliezer, and Chachamim against R. Meir. The Magid Mishneh says so in the name of the Ramban, and so wrote Tosfos (16a DH v'Rav), that Vestos are mid'Rabanan. Also the She'altos and R. Chananel say so. Therefore, if a woman did not check herself at the time of her Veses, and checked herself later and found herself to be Tehorah, she is Tehorah to her husband. However, it seems that l'Chatchilah she must check at the time of the Veses. Also Hagahos Maimoniyos and the Ra'avad say so. However, the Ran says that the Rif and Rambam hold that even though she should check herself at the time of the Veses, if the Veses passed and she did not check and she was not Margish, she is Tehorah without Bedikah. The Tur wrote Stam like Tosfos and the Rashba, who forbid until she checks.

Pischei Teshuvah (16): This refers to a proper Bedikah that enters holes and cracks.

Pischei Teshuvah (17): If there is a fixed hour for the Veses, she needs to check only then. If there is no fixed hour, or for the average Veses, she must check the entire day (or night). I.e. she inserts a tight fitting tuft there the entire time. If her Veses varies over two to three days, she must leave it in the entire time. It is like her Veses.

Shulchan Aruch (ibid.): Some say that she is forbidden until she checks if she has a Veses Kavu'a, or if it is day 30 even if it is not Kavu'a. This is the custom, and so it is below in 189:4.

Beis Yosef (DH u'Mihu): However, in Toras ha'Bayis the Rashba wrote that this is if she has a Veses Kavu'a. If she has a Veses that is not Kavu'a, even if she did not check at the time of the Veses, she is Tehorah and she need not check. As long as she was not Margish at the time of the Veses, even if she did not check, she has Chezkas Taharah, since she does not have a Veses that is not Kavu'a. The Isur of this Onah is only mid'Rabanan. The Tur says so at the end of this Siman, and the Magid Mishneh said so in the name of the Ramban. The Rashba said that this is for a Veses she'Einah Kavu'a less than the average Onah. The average Onah, which is 30 days, is like a Veses Kavu'a. Even if she was not Margish, she is forbidden until she checks and finds herself Tehorah.

Beis Yosef (DH v'Chasav Od): The Rashba says that if a woman has no Veses, day 30 is like a Veses Kavu'a for her. I say that the same applies if she has a Veses she'Einah Kavu'a more than 30 days. She must check on day 30. She need not check on the day of her Veses. Even though she is Tehorah without checking for a Veses she'Einah Kavu'a, she must refrain (from Bi'ah) at the time of the Veses.

Taz (12): Even if she did not check until much later, we do not say that the blood fell to the ground at the time of the Veses.

Shach (23): The Beis Yosef is lenient even if she did not check immediately. The Bach is stringent like the Mordechai concluded in the name of Re'em, Sefer ha'Terumah, and Sha'arei Dura in the name of Semak, that it is only if she checked immediately. If not, the Chazakah is that the blood comes in his time; the blood fell to the ground.

Taz (13): The Prishah asked that in Siman 186, we forbid a woman without a Veses to have Bi'ah without Bedikah. This is not difficult. A woman without a Veses normally see after a different interval each time. The Chazakah is that she sees, until she checks. Here, we discuss one who has a Veses like other women, just she changed and saw once after (e.g.) 25 days. She must be concerned another 25 days from now (lest she see then). Since she had no Hargashah that day, and did not check, she is Tehorah, since she is not established three times to see that day. It suffices to be concerned for the day l'Chatchilah.

Gra (20): The latter opinion learns from 'Chachamim say that she must check.' The first opinion explains that this discusses the time of the Veses. It learns from Rabah bar bar Chanah, who permits even if he returned after the Veses (he may assume that she is Tehorah), i.e. even right after the Veses, unlike Tosfos. R. Yochanan holds that Vestos mid'Oraisa, and the Halachah does not follow him.

Gra (21): If a woman has no Veses, her Onah (i.e. 30 days) is like a Veses, like Reish Lakish taught (15a, 9b). See Rashi (15a DH b'Soch, who says that Stam women see at the end of the Onah).

Pischei Teshuvah (19): The Acharonim say not to be concerned for this stringency.