If a pregnant woman has a menstrual flow within the first 3 months of pregnancy, must she observe the vestos resulting from this flow?
Answer:
During the first three months, it is correct to observe the vestos. However, after the third month is up, there is no longer an obligation to observe them, though some write that if there was a flow in the third month, it should be observed in the fourth.
Sources:
According to the Gemara and the classical poskim (including the Shulchan Aruch 184:37 and 189:36) a woman continues to have a menstrual cycle into the first three months of her pregnancy, and these will therefore be of consequence for her veses.
However, Rav Moshe Feinstein (Iggros Moshe Yoreh De’ah 3:52; 4:17) writes that in our days, a woman is considered clean of blood from the first months of her pregnancy.
According to this approach, a menstrual flow, even in the first three months, will be considered a mikreh, “by chance,” and will not obligate its days of veses for the next months.
However, because according to the reality described by Chazal the menstrual cycle continues at the beginning of pregnancy, and because this woman apparently fits the description, it is hard to rely on this for the particular woman, and therefore the vestos should be kept during the first three months.
In the fourth month there is no longer an obligation to keep the vestos, but Rav Nissim Karelitz (Chut Shani p. 43, siman 11) writes that it is preferable to keep the veses, after having the menstrual cycle during pregnancy.
Best wishes.
