A woman during labor may become a Nidah in any of the following scenarios:

1. Blood: If blood flows from her vagina, she is deemed a Nida. [Some write that this applies even by a Kesem, such as if she saw blood on a pad or colored undergarments, if she is in the middle of active labor.[2] However, some write that it follows the same laws as always, in which if the stain is found on a garment then it follows the general rules of a Kesem.[3]]
2. Contractions: Some Poskim[4] rule that experiencing a consistent amount of contractions every few minutes deems her a Nida.[5] Other Poskim[6], however, are lenient. Practically, if she has contractions every 3 to 10 minutes, or has extremely heavy and painful contractions, then she is deemed a Nida.[7] If, however, her contractions stop then she is not a Nidah.[8]
3. Birthstool/Mashber:[9] Some Poskim[10] rule if she is ready to go on the birth stool to give birth she is a Nida.[11] Other Poskim[12] rule she is not a Nida.
4. Can’t walk on own:[13] If she is unable to walk on her own due to the labor, and needs to sit on a bed or wheelchair, or have someone help her walk, then she is deemed a Nida. If, however, she is able to walk on her own and simply needs help going up and down the stairs then she is not deemed a Nidah.
5. Dilation:[14] If a woman has dilated due to the contractions, many Poskim[15] rule that she is a Nida. Some Poskim[16] rule that if she has dilated two fingers [4 cm], then she is a Nida. Other Poskim[17] however, rule that the amount of dilation which deems a woman a Nida is measured differently in every women, as some women have natural dilation of more than 4 cm in end of 8th month, and hence only if she is open and very close to birth is she impure. Furthermore, there are Poskim[18] who do not deem dilation as a factor for becoming a Nida.
6. Water broke:[19] If her water broke, then if there is blood in the water, then she is a Nida. If, however, she does not see blood in the water, then she is not a Nida.[20] However, some Poskim[21] rule that if her waters broke well within active labor, directly prior to birth, then she is impure
7. Mucus plug:[22] If the mucus plug came out as a thick substance then she is impure. If however it is a mere liquid, she remains pure if there was no blood in the liquid.
8. Stripping: Some Poskim[23] rule that membrane stripping to induce labor deems a woman a Nida.
9. Internal: If an internal examination was done past her cervix, she is deemed a Nida, as explained in the previous Halacha.

[1] See Taharah Kehalacha 11:1-3; Nitei Gavriel Nidda 3 Chapter 106; Toras Hayoledes chapter 19; Encyclopedia Hilchatit Refuit Leida p. 256

Background: These Halachas are not mentioned in this chapter in Shulchan Aruch. The Poskim rather compare this scenario to that of Shabbos regarding when a woman is considered endangered.

[2] Shaareiy Tohar 14:11; Nitei Gavriel 106:15; Toras Hayoledes 19:1; I had asked Rav Farkash regarding a case that she was opened 3 cm and he ruled to me that if she saw blood even on pad then she is Tamei if she is in middle of active labor.

[3] Beir Moshe 3:152; Nitei Gavriel 106:1

[4] Madnei Hashulchan; Nitei Gavriel Nidda 106:1; See also Shiureiy Sheivet Halevi p. 245 regarding that if there are very strong contractions or they are very close together we suspect her womb opened and released blood.

[5] The reason: As we suspect for the opening of womb.

[6] Rav Kohen denied this; Taharah Kehalacha 11:1; Darkei Tahrah [Eliyahu] 11 [p. 110]

[7] Nitei Gavriel Nidda 106:1 and 4

[8] Nitei Gavriel 106:16 and 19; However, see Toras Hayoledes 19:4 that custom of women from Poland is to be stringent

[9] See Shiureiy Sheivet Halevi p. 244 [194:2]

[10] Nachalas Shiva 9; Pleisi 194:1; Sidrei Taharah 194:25; Kreisy Upleisy 194:1; Beis Lechem Yehuda 194:1; Pischeiy Tehsuvah 194:8; Darkei Teshuvah 194:39; Tahara Kehalacha 11:1-2 footnote 1; Igros Moshe 2:75

[11] The reason: As they compare this to the law of Sakana of a Yoledes on Shabbos.

[12] Chavas Daas 194:1; Chasam Sofer 179; Teshuva Meahava 1:14; Darkei Tahrah [Eliyahu] p. 110; Rav Cohen

[13] Sidrei Taharah 194:25; Beis Lechem Yehuda 194:1; Igros Moshe 2:75; Shiureiy Sheivet Halevi 194 p. 245; Shevet Halevi 4:106; Taharah Kehalacha 11:1 based on above Poskim that are stringent; Nitei Gavriel Nidda 106:1 and 5

Other Poskim: Some Poskim rule that she is not a Nidda. [Darkei Tahrah [Eliyahu] 11 p. 110].

[14] See Taharah Kehalacha 11:4; Kaneh Bosem; Shiureiy Sheivet Halevi p. 244-245; Nitei Gavriel Nidda 106:1; Toras Hayoledes 19:1

[15] Chasam Sofer 179 brought in Pischeiy Teshuvah 194:8 implies if open even by a little dilation she is impure; Shevet Halevi 2:4; Taharah Kehalacha ibid rules that if the dilation is as result of the birth she is impure. He however does not say how many cem. Shiureiy Sheivet Halevi p. 244-245 [194:2-4] writes that so long as she can walk she is pure even if she is open 4-6 cm. However he also writes that if there are very strong contractions or they are very close together we suspect her womb opened and released blood.

[16] Nitei Gavriel Nidda 106:1; Rav Eliyahu; If it is under 2-3 centimeters is not considered dilation. [Shevet Halevi 4:106; Toras Hayoledes 19:1 based on Chabas Daas 194:1; Jacobs] Rav Kohen said that under 4 cm is ok, although by first birth any opening is deemed a Nida. Above 4 cm is always a Nida;

Other Poskim:

[17] Shiureiy Sheivet Halevi p. 244-245 [194:2-4]

[18] Beis Lechem Yehuda 194; Omitted in Igros Moshe ibid; See Chavas Daas 194:1; Pleisi 194:1; Nitei Gavriel 106:14

[19] See Nitei Gavriel Nidda 106:9; Encyclopedia Hilchatit Refuit Leida p. 258

[20] Taharah Kehalacha 11:1-3 footnote 2; Beir Moshe 3:152; Rav SZ”A in Nishmas Avraham 194:1; Darkei Tahara [Rav Eliyahu] p. 110; Shiureiy Sheivet Halevi  p. 245 is stringent if it happens very close to birth; However there are Poskim which are always stringent. [Rav Ashkenazi;Rav Cohen]

[21] Shiureiy Sheivet Halevi 194:2-4 [p. 245]; Shevet Halevi 194:2-4; Toras Hayoledes 19:5; Rav Ashkenazi Kfar Chabad; Rav Cohen

[22] See Nishmas Avraham Y.D. 194:183 in  name of Rav Noiyburt and Rav Elyashiv; Minchas Shlomo 2:72-38-39; Shiureiy Sheivet Halevy 194:2-4; p. 255; Teshuvos Shraga 188 footnote 125; Response of Harav Farkash to author; Nitei Gavriel 106:12

Answer of Rav Farkash: אם זה בסגנון של ליחה בלבד, אינו מטמא [כל עוד אין בו דם]. אבל אם הוא כמו גוש רירי, דעת פוסקי דורנו כפי שהובא בספר נשמת אברהם שאכן איננה טהורה. 

Other opinions: Some Poskim rule that she is pure. [See in length Rofei Uman p. 46; Shut Machon Puah Hirayon Leida p. 87-97]

Background: The consensus of the vast majority of Gedolei Haposkim is that if the mucus plug came out as a thick substance [i.e. Gush Riri] she is impure due to the rule of Ein Pesichas Harechem Beli Dam which means that whenever the womb is opened blood is always consequently expelled. If however it is a mere liquid she remains pure if there was no blood in the liquid. This is how I personally received to rule. However, there are Rabbanim who are lenient based on the following factors: a) The plug is found on the outer side of the cervix, by which many Poskim hold that the rule of “Ein Pesach Harechem Bli Dam” does not apply to it. b) the blood that is on it is blood of injury and not of the womb.

[23] Nitei Gavriel 106:13