Gemara

(Mishnah): If a man 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 - Question: Are Vestos mid'Oraisa or mid'Rabanan?

Answer (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.

(Rav): 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;

(Shmuel): She is Teme'ah. We assume that the blood came in its usual time.

Suggestion: Rav holds that Vestos are mid'Rabanan, and Shmuel holds that Vestos are mid'Oraisa.

Rejection #1 (R. Zeira): No, all agree that Vestos are mid'Oraisa;

Rav discusses one who checked herself within Shi'ur Veses (a very short time). Shmuel discusses one who checked herself after Shi'ur Veses.

(Rav Nachman bar Yitzchak): Indeed, 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.

(Abaye): We know R. Meir's opinion from a Mishnah!

(Mishnah - R. Meir): If a woman was hiding (from an invading army or bandits) and her Veses came and she did not check herself, she is Tehorah, for fear inhibits blood.

Inference: If not for the fear, she would be a Nidah. This shows that R. Meir holds that Vestos are mid'Oraisa.

Suggestion: Also the following Tana'im argue about whether Vestos are mid'Oraisa:

(Beraisa - R. Shimon ben Gamliel): If a woman sees blood due to a wound (in the area), even at the time of her Veses, she is Tehorah;

Rebbi says, if she has a Veses, she is concerned for it.

Rejection (Ravina): All agree that Vestos are mid'Rabanan. They argue about whether blood from the Makor (which is not Dam Nidah) is Tamei.

Rishonim

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

Ran (DH v'Garsinan): The Ra'avad says that Rabah bar bar Chanah and R. Yochanan argue, but both agree that Vestos mid'Rabanan. The Rif and Rambam hold that R. Yochanan holds that Vestos mid'Oraisa, unlike Rabah bar bar Chanah. We hold like Rabah, for the Sugya holds that Vestos mid'Rabanan. Also Rav agrees.

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: The Rambam derived this because Tum'as Vestos is mid'Rabanan. This is like me'Es la'Es (retroactive Tum'ah for 24 hours) in a woman without a Veses. A Beraisa (Nidah 6a) teaches that one who sees blood is Metamei retroactively food and drink and Kelim, but her count is in order (we are not concerned lest she became Nidah earlier) and she is not Metamei her husband. She counts only from when she saw. The same applies to Vestos, since they are mid'Rabanan. The Gemara suggested that Tana'im argue like Rav and Shmuel, but concluded that all the Tana'im agree that Vestos mid'Rabanan. Rashi explains that Rebbi is lenient. He teaches that if you are Metamei because she has a Veses, she should be Tamei for seven days. Since you do not say so, she is totally Tehorah. The Rambam holds like this.

Rosh (1:1): The Gemara asked whether Vestos mid'Oraisa. I.e. is it a tradition from Moshe from Sinai that the blood comes in its time? If so, when the Veses comes, we establish her to be Temei'ah even if she checked herself afterwards and found herself to be Tehorah. Or, is it mid'Rabanan, and if she found herself to be Tehorah, she is Tehorah? We hold like Rav Nachman bar Yitzchak, for he is Basra. He says that Rav and Shmuel argue about whether Vestos mid'Oraisa or mid'Rabanan. We hold like Rav in Isurim, therefore Vestos are mid'Rabanan. R. Yochanan agrees. Also, we hold like R. Yehoshua against R. Eliezer, and like Chachamim against R. Meir. Also, Ravina is Basra, and he struggled to say that Rebbi and R. Shimon ben Gamliel argue about whether the place of the Makor is Tamei, but all agree that Vestos mid'Rabanan. 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, p.1a column 2 DH v'Ishah, citing Re'em): If a woman has a Veses, all agree that she must check at the time of the Veses. The Halachah follows Rav, that Vestos mid'Rabanan. R. Zeira explains that Rav is Metaher if she checked herself within Shi'ur Veses, i.e. immediately, of her Veses. If she delayed after the Veses, even if she found herself Tehorah, she is Temei'ah, for Vestos mid'Oraisa. I.e. Chachamim relied on Vestos as if they were mid'Oraisa, for the blood comes in its time. Even though there are Amora'im who argue with R. Zeira, and say that Rav is Metaher even if she did not check immediately, we are concerned for R. Zeira.

Mordechai: The conclusion is that if a woman has a Veses, and she did not check at the time of the Veses [and not within Shi'ur Veses afterwards], even if she found herself to be Tehorah, she is Temei'ah.

Poskim

Shulchan Aruch (YD 184:9): If the Veses passed and she did not check and was not Margish (have her usual sensation when the blood leaves the Makor), she is Tehorah without checking. Some say that she is forbidden until she checks if she has a Veses Kavu'a.

Beis Yosef (DH u'Mihu): It is known that the Halachah follows Rav against Shmuel in Isurim. Tosfos, the Rashba and Rambam rule like Rav. The Magid Mishneh says that all Meforshim agree that Vestos mid'Rabanan. The Mordechai agrees, even though he wrote differently in the name of Re'em. It seems that all the Poskim disagree (with Re'em). Even if she delayed long after the Veses, if she checked and found herself Tehorah, she is Tehorah.

Note: Why does the Beis Yosef says that the Mordechai agrees that Vestos mid'Rabanan? Perhaps it is because even Re'em says that Rav agrees that Vestos mid'Rabanan, just Chachamim were stringent as if they were mid'Oraisa.

Bach (5): We hold that Vestos mid'Rabanan. All the more so, the Isur of Bi'ah Samuch l'Vestah (the night or day before her Veses) is only mid'Rabanan. Therefore, the Tur brings from the Ra'avad that if a woman is unsure whether she normally sees before or after sunrise, this is a Safek mid'Rabanan, and she must refrain only in the daytime (for Vadai she is Temei'ah then, whether she sees before or after sunrise). This is the Halachah. However, I say that it is proper to be stringent like the Ohr Zaru'a, and always forbid the day and (previous) night, due to the possibility of this mistake.