Question:

Does a woman become a Niddah if her mucus plug came out?

Answer:              

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.

Explanation: 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.

Sources: Stringent: 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; Lenient: See in length Rofei Uman p. 46; Shut Machon Puah Hirayon Leida p. 87-97; See regarding the general rule of Ein Pesichas Harechem Beli Dam: Admur Y.D. 188:8; Michaber 188:3; Niddah 21a; 66a; Nodah Beyehuda Tinyana 120; Chasam Sofer 179; Shiyarei Taharah 194; Kinas Sofrim 73; Beis Shlomo 39; Kreisi Upleisi; Avnei Nezer 224;Taharah Kehalacha chapter 7:2 footnote 12; Nitei Gavriel 80:8-9; Family Purity page 16