The law of Bedikos by a Hargasha of a pregnant woman:[1]

Initially doing Bedika even when pregnant: If a woman who is Mesulekes Bedamim [within 2 years of giving birth, or past 3 months of pregnancy] had a true Hargasha[2] then if she has yet to establish a Chazaka of three times that her Hargashas are of a pure color[3], then she is to check herself, just as we rule by any other woman. If she only felt a Zivas Davar Lach, then she is not required to check at all if she is over three months pregnant.[4]

Did not find anything on Bedika when pregnant:[5] If a woman who is Mesulekes Bedamim [within 2 years of giving birth, or past 3 months of pregnancy]  checked herself after feeling a true Hargasha and did not find anything [neither a pure or impure color], then she remains pure [unlike the ruling by a woman who is not a Mesulekes Bedamim[6], including a woman within three months of pregnancy in which case we are stringent[7]].

If she did not check herself at all when pregnant:[8] However, if she did not check herself at all after feeling a Hargasha, then she is impure even if she is a Mesulekes Bedamim as we rule by any other woman. [Practically, she is to check herself as soon as she remembers, and if she does not find a impure color, she is pure.[9]]

Establishing a Chazaka while pregnant:[10] Tzaruch Iyun if a Mesulekes Bedamim [within 2 years of giving birth, or past 3 months of pregnancy] is able to establish a Chazaka by performing three Bedikas after feeling a Hargasha. [Prior to being three months pregnant, she may perform the Bedikos to establish a Chazaka, just like any other woman.]

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[1] See Shiureiy Shevet Halevi 190:14; Taharah Kehalacha 1:8; Kitzur Dinei Taharah 1:7; Nitei Gavriel 70

[2] See Michaber 183:1; Taharah Kehalacha 1; Kitzur 1:7

What is a Hargasha? A Hargasha refers to a woman feeling a flow of liquid in her vaginal cavity. Biblically, a woman only becomes a Nida if she feels her womb open and release blood from the womb to the vaginal cavity [past the cervix].  In such a case, even if merely a drop of blood was released, she is deemed a Nida. If she only feels the flow of blood within the vaginal cavity, and did not feel it leaving the womb into the cervix, then there is a dispute as to whether such a feeling of blood makes her Biblically into a Nida or not. This form of flow is referred to as a “Zivas Davar Lach.” [See Pischeiy Teshuvah 183:1 for a dispute between the Nodah Beyehuda 55 which is stringent to view it as a Hargasha [Biblical] and the Chasam Sofer 145 which is lenient to view it as a Kesem.] If she did not feel the flow of blood at all, then even if she finds blood later on, on her under garments, and knows it came from her womb, she nevertheless remains Biblically pure. However, Rabbinically she becomes impure in certain situations. This latter form of impurity is referred to as “Kesamim”.

[3] Doing a Bedikah every time she feels a Hargasha until verifies 3x that it is merely mucous:  Whenever a woman who is pure, or within her seven clean days, feels a discharge from her vagina she must immediately check herself with a Bedika cloth, following all the laws of how a Bedika is to be done, in order to verify whether the flow was blood or merely mucous. [See Michaber 190:1 in name of Terumas Hadeshen] If she finds any blood on the cloth she is impure. If she finds mucus or other white texture that has some thickness, she is pure. [See Michaber 188:1]  If she did not find any blood and also did not find thick mucus, or if she did not check herself at all, then the ruling depends on the type of Hargasha she felt, whether it was a Hargasha from the womb or from her vaginal cavity. If she felt a Hargasha from the womb she is impure if she did not check herself immediately, or checked herself but did not find thick mucus. [Michaber 190:1 in name of Terumas Hadeshen] If it was a Hargasha from her vaginal cavity, she is pure even if she did not check herself at all, or did but did not find thick mucus. [Divrei Chaim 2:66; Shiurei Shevet Halevi 190:1]

Establishing a Chazaka:  If she has checked herself three times immediately after having three separate flows [of the same type ], one time after each flow, and found the same substance of mucus or other white substance, then she has readily verified that this type of flow is merely mucus and the like, and she no longer needs to check herself anymore when she feels such a flow.  However if she feels a different type of flow then the previous ones which she already verified, she must check herself to verify whether this flow is blood or mucus. Thus practically as soon as the Kalah has begun counting her 7 clean days prior to the wedding, she must be aware of if she is having a flow, and if so to subsequently do a bedikah to verify whether it is blood or mucus.  It is thus advisable that starting from that point she should begin carrying Bedika cloths with her so they become readily available. This of course continues to apply likewise on the day of the wedding, until intercourse and on the seven clean days after her 1st intercourse and so on when she becomes pure, until she verifies three times that the flow is merely mucous, as said above that this bedikah is an obligation until the 3 time verification is done. [See Divrei Chaim 1:34; 2:66; Mahrsham 1:188; Chochmas Adam 113:3; Pischeiy Teshuvah 190:3; Shiureiy Sheivet Halevi 190:11; Taharah Kehalacha 1]

[4] Imrei Yosher 1:182; Levushei Mordechai Kama Y.D. 116; 105; Tinyana Y.D. 92; Mahrshag 1:10; Nitei Gavriel 70:4 that seemingly even within the three months she is not required to check

[5] See Michaber 183:1; Admur 190:2; Toras Hashelamim 190:2; Sidrei Taharah 190:3; Chochmas Adam 113:2; Aruch Hashulchan 183:13; Lechem Usimla 183:8; Tzemach Tzedek Piskeiy Dinim; Neta Shoreik 51; Taharah Kehalacha 1:8; Kitzur 1:7; Shiureiy Shevet Halevi 190:14; Nitei Gavriel 70:1 footnote 1 and 3

Other opinions: Some Poskim rule that even a Mesulekes Bedamim is impure if she feels a Hargasha and did not check and find a pure color. [Tiferes Lemoshe; Hagahos Rav Akiva Eiger; Tiferes Tzevi 190; See Taharah Kehalacha ibid footnote 24; Nitei Gavriel 70:1 footnote 2]

[6] Michaber 190:1 in name of Terumas Hadeshen

[7] Peri Hasadeh 2:48; Nitei Gavriel 70:3

[8] Shut Tzemach Tzedek 134; Beis Shlomo 2:33; Taharah Kehalacha ibid

Other opinions: See Peri Deiah

[9] Shiureiy Shevet Halevi 190:14; Taharah Kehalacha ibid footnote 24

[10] Seemingly, this is dependent on the dispute of whether any leniency is given to a Mesulekes Bedamim regarding if she checked and did not find any color at all. Vetzaruch Iyun.