Yom Chashash-Vestos: Suspecting for the Vestos [i.e. Onah Beinonis, Veses Hachodesh, Veses Haflaga] while Pregnant:[1]

The following law will discuss whether a woman who is pregnant needs to keep any of the normally required Yimei Chashash [i.e. Onah Beinonis, Veses Hachodesh, Veses Haflaga] that are marked on the calendar.

Within the first three months of pregnancy:[2] Within the first three months of pregnancy [which is counted from the night of Mikveh[3]], the, couple must abide by all of the Yimei Chashash that are marked on their calendar from all previous periods. Certainly, if she became a Nida within her first or second or third month of pregnancy, then they must mark the various Yimei Chashash on the calendar and follow them if they fall within the first three months. Likewise, within the first three months of pregnancy she can establish a Kevius Veses [which she would suspect for until she enters her fourth month, as explained next].[4]

After three months of pregnancy: Once a woman enters into her fourth month of pregnancy, then she enters the status of a Mesulaekes Bedamim and she no longer needs to suspect for any of the Yimei Chashash that have been marked on the calendar from previous periods that took place within the first three months of pregnancy or prior.[5] Thus, any periods that she had within her first three months no longer need to have any of their Yimei Chashash suspected for in her fourth month and onwards. This applies even towards a Veses Kavua, and even if she sees blood on that same patters while pregnant it only has the status of a Veses Sheiyno Kavua.[6] However, if she sees blood after entering her fourth month and onwards, then it must be marked on the calendar and have all of its Yimei Chashash [Onah Beinonis, Veses Hachodesh, Veses Haflaga] properly guarded even while pregnant and under the status of Mesulekes Bedamim.[7]

Kevius Veses-Saw blood on same date/pattern three times: A woman who is Mesulekes Bedamim cannot establish a Kevius Veses.[8] Thus, a woman who is within 24 months of birth, or past three months of pregnancy, and sees her period in a pattern that creates a Veses Kavua, suspects for all the three suspicions of Hachodesh, Beinonis, and Haflaga.[9] However, within the first three months of pregnancy [which is counted from the night of Mikveh], a woman is not a Mesulekes Bedamim and can establish a Kevius Veses [which she would suspect for until she enters her fourth month, as explained above].[10]

[1] See Michaber Y.D. 184:7; 189:33-34; Admur 189:113-114; Tahara Kehalacha 1:8; 24:97; Encyclopedia Hilchatit Refuit Vol. 3 p. 22; Nitei Gavriel Nidda Vol. 2 Chapters 103-105

[2] See Nitei Gavriel 104 in detail

[3] Avodas Hagershoni 21; Nitei Gavriel 103:8

[4] See Nitei Gavriel 104:2

[5] Michaber Y.D. 189:34

[6] Michaber 189:34; Admur 189:115; Nitei Gavriel 105:2

[7] Michaber 189:33; Admur 189:114 [including Beinonis]; Chavas Daas 189:7; Maharam Padwah 5; Misgeres Hashulchan 158:29; Maharshag 3:54; Nitei Gavriel 105:1

Other opinions: Some Poskim rule that one is not required to suspect for the Onah Beinonis when she sees blood as a Mesulekes Bedamim. [Sidrei Taharah 189:36; Pischeiy Teshuvah 189:30; Nitei Gavriel 105:1]

[8] Michaber 189:33

[9] Admur 189:115; Derisha; Toras Shelamim 184:16; Taharah Kehalacha 24:98-100 footnote 154

Other opinions: Some Poskim rule that she can be Koveia a Veses just like any other woman, and in such a case must only suspect for those times. [Sidrei Taharah 189:36; Chavos Daas Chidushim 189:50; Pischeiy Teshuvah 184:14; Emek Sheila 23; Suga Beshoshanim Vestos 10:2 [siman 61]; Shiureiy Sheivet Halevi p. 152; Nitei Gavriel 105:4]

[10] See Nitei Gavriel 104:2