The case of a pregnant woman is similar to the case of the menopausal woman in many ways, but not in suspecting a non-fixed cycle.  The older woman does not need to suspect these dates and a pregnant or nursing woman does.  The reason is that an older woman has lost her chazaka of having flows and we therefore do not anticipate any future bleeding.  The pregnant woman, however, normally will have a flow but it is suspended because of her physical state.  Therefore, she does not lose her chazaka as a woman who expects bleeding and she therefore must suspect the vesses she'aino kavua rules.  She also then has the benefit of having a cycle uprooted after one future period rather than after the 3 required for a vesses kavua.

The rule of three months was developed because pregnancies were verifiable only by seeing the woman look pregnant.  Today, women can find out as quickly as 15 days after conception that they are pregnant.  Nevertheless, we do not extend the leniency of Seif 33 back to the contemporary point of learning about pregnancy.  The exemption of a pregnant woman from vesses kavua is a leniency in the law that contemporary poskim have chosen not to extend.

As a point of interest the Ramban disagrees with the idea that a woman cannot establish a vesses kavua during pregnancy.  The Tur follows the lead of the Ramban, but the Shulchan Aruch goes in the opposite direction. The Bach challenges the conclusion of the Shulchan Aruch and maintains that normative halacha should  follow the Tur.  Shach (73) explains that there several reasons why the halacha follows the lenient view of the Shulchan Aruch.  Most significantly, he asserts that it is evident that the Tur never saw the original contrary position developed by the Rashba.  The general principle is that although we normally follow the decisions of the Achronim and therefore the Shulchan Aruch should have followed the Tur, if the Acharon did not have all of the sources available to him, we can assume that if he did in fact have all of the sources he would have come to a different decision.  Second, the Ramban is a single view against many.  Third, the Rtiva points out that the accepted custom was to follow the practice noted in the Shulchan Aruch.  There is a general principle that if there is an area in which halacha is unclear or is debatable, we follow the accepted custom, if one exists.  This idea partially shows the power of minhag in determining practice and why one does not change one's minhagim casually.  Finally, the Shach notes that the whole of idea of vesses is Rabbinic, and we are therefore lenient in areas of doubt so we suspend the stringencies of hte vesses kavua for the period of pregnancy.

Another idea is why the halacha chooses to tell us about pregnant and nursing women separately.  Clearly, one cannot be nursing without being pregnant and the end result of a normal pregnancy is nursing.  The Gemara (Niddah 9a) discusses an argument between R. Meir and the Rabbanan as to whether a woman doesn't bleed during the 24 months because of the effect of producing milk or because of the damage done to a woman's body during delivery.  The Halacha follows the latter, albeit scientifically incorrect, view.  Based on this the Badei HaShulchan suggests that a woman who gives birth by caesarean does not experience the same physical injury that she would by ordinary delivery.  He wants to say that it is possible that women who give birth by c-section do not have the 24 month period allotted to them!

Because women today typically experience a resumption of their periods long before 24 months have past and even while nursing, this halacha has minimal application and women do in fact establish vestos during the 24 months period following childbirth.

The end of Seif  33 tells us that a pregnant woman must suspect various days as though she has a vesses sheaino kavua.  The Sidrei Taharah suggests that a woman need not suspect the onah beinonis, since we do not expect her to have menstrual flows like the "average" woman.  Shulchan Aruch harav disagrees and insists that a woman must observe the onah beinonis.  One should seek competent halachic guidance if one is in this situation.