Gemara

(Rav Papa): If a woman normally sees every 22 days, she is forbidden on days 22, 44,... If now she saw on day 27, she is also concerned for 27 days from her Veses (day 22), i.e. day 49.

Rejection (Rav Huna brei d'Rav Yehoshua): Perhaps she is concerned for days 22 and 27 from her sighting (on day 27).

Support: If a chicken normally lays an egg every other day, and once it did not lay until the third day, surely, we expect it to next lay two days from this egg, not from when it should have laid (yesterday). Likewise, a woman expects to see after her Veses (i.e. normal interval between initial sightings of Dam Nidah) after this sighting!

(Rav Huna brei d'Rav Yehoshua): If a woman became a Nidah on Rosh Chodesh, and on the next Rosh Chodesh, and on the 25th of that month, and saw five (or six) days later on Rosh Chodesh, R. Yochanan says that she fixes a Veses. On the 25th she merely saw extra early blood due to her primary sighting on Rosh Chodesh.

64a (Rav): If a woman saw on the 15th of the month, on the 16th of the next month and on the 17th of the third month, she established a Veses of Dilug (skipping).

(Shmuel): She does not establish a Veses until she sees three times after the same skip.

Both of them hold like R. Shimon ben Gamliel, who says that Chazakah is established after three times;

Rav counts the 15th as one of the three times. Shmuel does not count it, for it did not come after the skip (that will be her Veses).

Question (again Rav - Beraisa): If a woman normally sees on the 15th, and she saw on the 16th, both of these are forbidden. If she saw (next month) on the 17th, the 16th is permitted, and the 15th and 17th are forbidden;

If she saw (next month) on the 18th, all these days are permitted, only (the day) after the 18th is forbidden.

Answer: Since she normally used to see on the 15th, it does not count towards a Veses of Dilug.

Question: The answer is obvious. Why did the questioner ask?!

Answer: One might have thought that after seeing twice on other days, she is no longer concerned for the 15th. The Beraisa teaches that this is not so.

Question (against Shmuel - Beraisa): If a woman saw on the 21st of the month, and on the 22nd of the next month, and the 23rd of the third month, she fixed a Veses;

If she skipped (in the third month) and saw on the 24th, she did not fix a Veses.

Answer: The case is, she normally sees (and saw the previous month) on the 20th.

Support: The Tana normally begins with the 20th (to teach as concisely as possible). Presumably, he began with 21 because she already saw on the 20th last month.

Bava Kama 37b: If an ox gored on the 15th, 16th and 17th of three consecutive months, Rav and Shmuel argue about this. (The Gemara then cites their argument in Nidah.)

Rishonim

Rambam (Hilchos Isurei Bi'ah 8:6): If a woman saw on Nisan 15, Iyar 16, Sivan 17, and Tamuz 18, she fixed a Veses. If in Tamuz she saw on the 17th, she did not yet fix a Veses. Every say that she sees, she is concerned for it for the future. Once that day comes and she does not see on it, that day is uprooted. We require three times (without seeing) to uproot a Veses only if it was established three times.

Rosh (9:3 (the second)): Rav and Shmuel argue about whether the 15th is part of the count. Rav counts it only when her sightings are based on the days of the month. However, if she first saw on Rosh Chodesh, and then 15 days later, and then 16 days from that, Rav agrees that she did not fix a Veses, for the first sighting does not count. This is clear from Bava Kama. It compares goring on the 15th, 16th and 17th to the argument here. Even though the skip between sightings is not equal, for the months alternate between full (30 days) and lacking (29 days), it depends on the Shofar (used to announce Rosh Chodesh). R. Chananel and the Ra'avad rule like Rav, for we hold like Rav (against Shmuel) regarding Isurim. The Rambam rules like Shmuel, for the Gemara challenged Shmuel from a Beraisa and answered this, and said that presumably the answer is true (and the Beraisa is unlike Rav). The Ramban agreed.

Rosh (Bava Kama 4:4): All agree that if she saw three times on Rosh Chodesh or three times on the 15th, she fixed a Veses, since the month causes her to see.

Beis Yosef (YD 189 Sof DH u'Mah she'Chasav Hacha): We learn from Nidah 39b.

Rosh (ibid.): If she saw today, and after 20 days, and after 20 days, and after 20 days, she fixed a Veses after the third sighting after 20 days. All agree that the first sighting does not count. They argue about Dilug (skipping days of the month). Their primary argument is about Vestos. The Halachah follows Rav in Isurim. Some say that since they did not argue about goring, just the Gemara said that they would argue like they argue about Vestos, also here (regarding goring) the Halachah follows Rav. I agree. The Gemara rules like Shmuel in monetary laws because he regularly judged them and was meticulous to descend to their depth and reach the Emes. We rule like Rav in Isurim, for he was meticulous about Isur v'Heter. Since they argue about Vestos, and the same reason applies to both of them, we rely on Rav for both of them. Had they argued only about goring, we would have ruled like Shmuel for both of them.

Note: Had they argued about goring, a monetary law, perhaps Shmuel would have been more meticulous and concluded like Rav.

Poskim

Shulchan Aruch (YD 189:7): How does one fix a Veses of the Chodesh through Dilug? If she saw on Nisan 15, Iyar 16 and Sivan 17, she does not fix a Veses until she sees on Tamuz 18. The first sighting does not join, because the intervals were not equal. However, if she had a Veses beforehand, and then she changed and saw three times b'Dilug, she fixed a Veses.

Be'er ha'Golah (11): This is like the Rambam, Ramban and Rosh according to the Tur, who rule like Shmuel. The Rosh in Bava Kama rules like Rav, and so do the Tur's Kitzur Piskei ha'Rosh there.

Taz (9): The Tur wrote that the Rosh rules like the first opinion, i.e. Shmuel. This is difficult, for in Bava Kama he rules like Rav!

Taz (7): E.g. she used to see every 29 days, and then she saw after 30 days, and then after 31, and then after 32. Also from her first two sightings (after 30 and 31 days) we know that she (now) sees based on the Dilug.

Shulchan Aruch (ibid.): This is because even the first sighting was b'Dilug, for she skipped her normal Veses.

Taz (8): E.g. she used to see on the 20th, and then saw on the 21st, 22nd, and 23rd. The 21st counts, for also it was through an even skip like the others. This is no Chidush. The Gemara said so merely to answer for Shmuel. Or, perhaps it is a Chidush that the first sighting on the 20th counts even if she used to see that day.

Shulchan Aruch (ibid.): Some say that even if she saw only on Nisan 15, Iyar 16 and Sivan 17, she fixed a Veses and she is concerned for Tamuz 18 and Av 19, and so on forever. One should be concerned for their opinion.

Beis Yosef (DH u'Mah she'Chasav Rabeinu): The Tur says that the Rosh concludes like this (like Shmuel). This is not clear from the Rosh's words. However, after saying that R. Chananel and the Ra'avad rule like Rav, he brought that the Rambam rules like Shmuel, for the Gemara challenged Shmuel from a Beraisa..., and the Ramban agreed. Since he brought their opinion last, this connotes that he holds like them. It seems that the Rashba rules like Shmuel, but at the end he said that one should be stringent to be concerned for the opinion of R. Chananel and the Ra'avad.

Bach (9): What is the Chidush that she is concerned for Tamuz 18 and Av 19? According to Rav, she fixed a Veses of Dilug in the month (to see one day later each month)! It seems that the Chidush is that she is concerned only for Tamuz 18, but not for the Tamuz 17, even though it is 31 days after Sivan 17, just like that day was 31 days after Iyar 16. Since she fixed a Veses of Dilug, she is not concerned for any other Veses.

Bach (12 DH Kasav): There is a stringency also according to the first opinion, i.e. she is concerned for two Vestos, for the old Veses is not uprooted until she fixes a new Veses! Indeed, the Rashba is concerned for the stringencies of both opinions. He initially thought to rule totally like Shmuel, therefore he said that one should be stringent also like R. Chananel and the Ra'avad, who rule like Rav.

Shach (19): The Bach says that we are stringent for both opinions. She is concerned for the Veses of Haflagah (equal intervals between sightings), which she established twice, and the Veses of skipping (days in the month), like the first opinion. Also the Ra'avad says so. Therefore, she is concerned also for Tamuz 17.

R. Akiva Eiger: The Shach is difficult. She did not see twice after the same interval, since Nisan is full and Iyar is lacking! The Bach means that (according to Rav) she is not concerned for the 17th of Tamuz, even though the skip between Iyar 16 and Sivan 17 is the same (31 days) as that between Sivan 17 and Tamuz 17. According to Shmuel, she is concerned for it, for she did not yet fix a Veses. Actually, according to Shmuel in any case (i.e. even if she had not seen on Iyar 16) she must be concerned for Tamuz 17, since it is the same day of the month as her sighting the previous month (Sivan).