Gemara

7a (Mishnah): A nursing woman is Dayah Sha'atah until she weans her baby;

R. Meir says, if another woman is nursing her baby, or if she (prematurely) weaned him, or he died, the 24 hours apply to her;

Chachamim say, she is Dayah Sha'atah (for the normal period of nursing, 24 months).

R. Eliezer says, any woman who did not see blood for three periods is Dayah Sha'atah.

R. Yosi says, a pregnant or nursing woman is Dayah Sha'atah after three periods without blood.

She is Dayah Sha'atah the first time she sees. The 24 hours apply to the second sighting;

If her first sighting was due to Ones, she is Dayah Sha'atah for her second sighting.

10b (Rav): This applies to all the women of the Mishnah.

(Shmuel): It applies only to Besulah and Zekenah (she ceased having menstrual periods), but a pregnant or nursing woman is Dayah Sha'atah as long as she is pregnant or nursing.

Tana'im argue like Rav and Shmuel;

(Beraisa - R. Meir): If blood was flowing from a pregnant or nursing woman she is Dayah Sha'atah as long as she is pregnant or nursing;

R. Yosi, R. Yehudah and R. Shimon say, she is Dayah Sha'atah the first time she sees. The 24 hours apply from her second sighting and onwards.

Rishonim

Ba'alei ha'Nefesh (Sha'ar Tikun ha'Vestos 1): If a woman who does not see blood during pregnancy and nursing returns to see afterwards, if she returns to her initial Veses even once, she fixed her Veses and needs (to pass the Veses without blood) three times in order to uproot it. Further, even if she returned to another Veses, she is like one who normally sees according to a Veses and changed her Veses. She needs three times in order to uproot the first Veses. This is a stringency of the interruption of nursing. For other interruptions, if three periods passed without any blood, and after three periods she changed to another day, she is concerned for only for the new day, but not for the old day. After the interruption of nursing, even if she began a new Veses, she is concerned for both of them. We learn from me'Es la'Es. If an old woman ceased seeing for three periods, and then saw, she is Dayah Sha'atah for the first new sighting. After the period of nursing ends, she is concerned for me'Es la'Es even on her first sighting. This shows that the Veses remains in its place and the blood returns to its place. The blood went away only temporarily. Even though this is not a clear proof, we can learn from reasoning, that for any interruption due to a reason, if the reason goes away, she returns to her Veses.

Rebuttal (Hasagos Ba'al ha'Ma'or 9): According to the opinion that any woman who had three periods without blood is Dayah Sha'atah, the same applies to a pregnant or nursing woman after she finishes pregnancy or nursing, since three periods passed. The Gemara said that she is Dayah Sha'atah during the entire pregnancy and nursing (10b). This was not to exclude afterwards. Rather, it includes all sightings during this time. This teaches unlike Rav, who says that all the women of the Mishnah resume retroactive Tum'ah after they see again. Really, also afterwards Dayah Sha'atah. If any woman had three periods without blood, Dayah Sha'atah. We do not say that the blood returned to its place. Further, one cannot bring a proof from retroactive Tum'ah to fixing a Veses. They are opposites! Our Tana holds that any woman with a Veses, Dayah Sha'atah. Even though R. Eliezer disagrees, he can say that the blood returned, but the Veses did not. Also, the Ra'avad says that whenever the blood left due to a reason, when the reason goes away, the Veses returns. The limbs of a pregnant or nursing woman are perturbed, and return only after 24 months. If so, if a woman had no Veses, after pregnancy and nursing the blood returns, and she has Tum'as me'Es la'Es. A Beraisa teaches that one who sees after Dam Tohar is Dayah Sha'atah. According to Levi, after Yemei Tohar, the source of Tamei blood is opened again. She should have Tum'as me'Es la'Es! Even though the Halachah follows Rav, Rav and Levi did not argue about this. Since Levi argues with the Ra'avad's rule, it is wrong.

Rashba (Toras ha'Bayis, brought in Beis Yosef YD 189 DH Ubrah): If a pregnant or nursing woman were (considered to be Mesulekes Damim, i.e. one who does not see blood) only after three periods without blood, when she returned to see three times, she would be Metamei me'Es la'Es, for she returned to their initial status. We say so about an old woman who had three periods without blood, or a girl before the age to see blood. If also a pregnant woman (were Mesulekes Damim) only after three periods without blood, when she returned to see three times, she would be like all women. Why did Chachamim say that she is Dayah Sha'atah the entire time of pregnancy and nursing?

Poskim

Shulchan Aruch (YD 189:34): After the days of pregnancy and nursing end, she returns to her first Veses. If she had a Veses of days, e.g. she used to see on Rosh Chodesh, she is concerned for the first Rosh Chodesh that comes. The same applies to all similar cases. The same applies if she had a Vest ha'Guf or for a known time.

Beis Yosef (DH veha'Ramban): The Ramban says that she returns only after seeing once, even if it was not at the time of her Veses. I.e. even though she saw not at the time of her Veses, she returns to her initial Veses and she is concerned for it until she uproots it three times. This is like a compromise between the two reasons. Since until now she was Muchzak to be Mesulekes Damim, she does not leave her Chazakah until she sees once. One sighting suffices to establish her to be one who sees blood. The Chazakah is that she returns to her initial Veses. Even though she saw not at the time of her Veses, she is like one who used to see on the 15th, and she saw on the 20th. Both of these are forbidden until it is uprooted three times.

Note: Only something established through three times requires three times to uproot it.

Shach (74): Even if she did not see on Rosh Chodesh, she is concerned for the second and third Rosh Chodesh. It is not uprooted in less than three times.

Shach (75): The Tur brought from the Ramban that she returns only after seeing once, but then she returns even if she does not see at the time of her Veses. Also the Beis Yosef brought like this from the Ra'avad. Why did the Shulchan Aruch omit their opinion? It seems that the Tur agrees. Vestos are mid'Rabanan, so we follow the lenient opinion!

Pischei Teshuvah (32): Noda bi'Yehudah (2 YD 86) says that this refers to fixed Veses. If she had a Veses that was not fixed (three times), since she became pregnant are her blood ceased, after nursing she is not concerned for it at all.

Shulchan Aruch (ibid.): However, if she had a Veses of skipping (a fixed number of days between periods), she cannot be concerned for it until (after) she returns to see. Once she returns to see, even once, she is concerned for the amount she used to skip (between sightings).

Beis Yosef (ibid.): The Ra'avad says that after pregnancy and nursing, even if she saw on a new day, she returns to her initial Veses. Regarding other interruptions, she is concerned for only for the new Veses. Some say that after nursing, she is immediately concerned for her first Veses even before she sees. This applies only to a fixed day of the month. It does not apply to a Veses of intervals between periods. Presumably, in both cases she is concerned only after she sees. The concern is only mid'Rabanan. As long as she did not see, we say that perhaps her blood did not return yet.