The Sifte Kohen (YD beg. §371) quotes the Rokeach saying that the wife of a Kohen may enter under the same “roof” as a corpse since there is a double-doubt: the newborn may be a stillborn and, even if it is not, it may be a female. The question was posed to Radbaz (§200): Why is such reasoning necessary, this is a case of tumah balu’ah? Radbaz therefore suggests that Rokeach was concerned with a situation where the woman is close to giving birth and we are suspect that the newborn might begin to exit the body whereby it is considered born and no longer balu’ah and so necessitated his “double-doubt” reasoning (cf. Chazon Ovadiah - Avelut vol. 2 pp. 63-68 quoting numerous authorities who [implicitly] agree that the fact of tumah balu’ah is alone sufficient reason to be unconcerned with a fetus contracting tum’ah, except that the double-doubt is necessary for a more extenuating circumstance).
As such, I believe the reason for a pregnant wife of a Kohen to be stringent and not enter a cemetery probably stems from esoteric reasons (cf. my comment to the question), unless the case in question is concerning a woman who has a chance of giving birth while visiting in the cemetery.