Bedikos required before and after Tashmish:[1]

If a woman does not have a set Veses [i.e. Veses Kavua], and is not Mesulekes Bedamim, then the couple is obligated to do a Bedika three times [i.e. Chazaka] before and after intercourse, one Bedika before and after three marital relations. The reason this is done is to verify that the intercourse does not cause blood to discharge from her womb. Once this has been done three times and the wipes have been found to be clean, then no more Bedikos are required.

Must she do Bedikos while pregnant:[2] A Mesulekes Bedamim [within 2 years of giving birth, or past 3 months of pregnancy]  is not required to perform the above Bedikah before and after Tashmish even if she has yet to establish a Chazaka, and she is past her Veses Haflaga. [Prior to being three months pregnant, she is required to perform the Bedikos to establish a Chazaka, just like any other woman, and this time-period is the most opportune time to establish a Chazaka, as explained next.[3] However, some Poskim[4] rule that one may be lenient to not do any Bedikos before and after Tashmish while pregnant, even within the first three months. Practically, one may be lenient in a time of need to suffice with a mere wipe.[5]]

Establishing a Chazaka while pregnant:[6] A Mesulekes Bedamim [within 2 years of giving birth, or past 3 months of pregnancy] is not able to establish a Chazaka by performing three Bedikas before and after intercourse. [Prior to being three months pregnant, she may perform the Bedikos to establish a Chazaka, just like any other woman. Furthermore, according to the approach that requires the Bedikos to be done past the safe days, the most opportune time to do Bedikos is in the 1st year of marriage, after seeing three Haflagos, while pregnant but within three months of pregnancy.[7]]

[1] See Michaber 186:1-3; Admur 186:8; Chasam Sofer 186; Lechem Visimla 186:4; Igros Moshe Y.D. 3:50; Shevet Halevi 7:145; Kitzur Dinei Taharah 13:7; 10:1; Taharah Kehalacha 23; Nitei Gavriel 64:11; 103:3-10

[2] Admur 186:7; Maharam Padwah 25; Toras Hashlamim 186:3; Sidrei Taharah 186:4; Pischeiy Teshuvah 186:4; Lechem Visimla 186:6; Taharas Yisrael 186:9; Tzemach Tzedek Miluim C.M. 2; Taharah Kehalacha 23:11; For definition of Mesulekes see Taharah Kehalacha 1:8; Nitei Gavriel 64:13; 103:3-10

[3] Aruch Hashulchan 184:35; Nitei Gavriel 103:3 footnote 10

[4] Beir Moshe 1:48; Poseiach Shaar 24:14; Shevet Halevi 3:116; Chazon Ish; Nitei Gavriel 103:3 footnote 7 in name of Rav Moshe Feinstein and other Rabbanim

[5] Shiureiy Shevet Halevi 186:2; Taharah Kehalacha 23:11

[6] Nitei Gavriel 64:14; Taharah Kehalacha 23:7 and 11; See regarding that the checks are only valid when done on unsafe days: Michaber 186:1-3; Admur 186:8; Chasam Sofer 186; Lechem Visimla 186:4; Igros Moshe Y.D. 3:50; Shevet Halevi 7:145; Kitzur Dinei Tahrah 13:7; 10:1; Taharah Kehalacha 23:3-7; Nitei Gavriel 64:11

[7] Taharah Kehalacha 23:7 and 11