I'm going to say that whoever wrote the wikipedia article did not understand the sources they were reading. First let me state the noticeable problems.
According to Halakha a woman may not tell her husband that she is Tameh when she is not (Even HaEzer 77 and Nosei Kelim). It would give her the status of a moredet, which is a person (a mored in the case of a man) who refuses to have marital relations with their spouse.
Saying a bracha at the mikvah would then result in a brakha l'vatala (a blessing said in vain), as the woman has no obligation to go to the mikvah.
Now let us look at the actual halakha. Regarding a pregnant woman the Shulachan Arukh YD 189:33&34 says:
A pregnant woman, after three months into her pregnancy, and a nursing
woman all twenty-four months after the birth of the newborn do not
establish a Vesses(a regular start of her period). This is valid even if the child died or if she
stopped nursing him since her periods are on hold during the all
pregnancy and all twenty-four months. A pregnant woman once the
pregnancy is noticeable and a nursing woman all twenty-four month do
not suspect their previous Vesses. Even if she had a Vesses Kavua and
it turned up during this time she is not required to make a Bedika and
is permissible to her husband. Even if they have a heavy discharge and
see in the very Onos that they are used to see, it is only a
coincidence.
Now regarding a woman who has a regular cycle the Shulhan Aruch YD 184:2 says:
At the time of her Vesses, he must separate from her for one ona; not
from all physical closeness, but only from relations. If the Vesses is
during the day, he must separate from her the entire day even if the
Vesses is at the end of the day, and he is immediately permitted that
coming night. Similarly, if it is at the beginning of the day, he’ll
separate the entire day but is permitted the entire previous night.
The same rule applies if it (the Vesses) is by night, he’ll separate
the entire night but will be permitted the previous and coming day.
(All this applies) whether she established the Vesses in three times
or only in one time.
So according to these two halakhot it is necessary to separate for one ona (12 hour period according to the proportional hour) during the ona that she is expected to have her cycle. The Or Zarua (which coincidentally is what most Chassidim hold by, and thus is pertinent to our discussion) on the Gemara Nidda 63b holds (depending on how you read him) that either they must separate for the ona she expects and the previous one as well (a 24 hour period; Chabad custom see Shulhan Arukh HaRav ad loc) or for the ona before and after the ona she expects which would make a 36 hour period (Satmar and Gerrer custom relying on my see Ohel Shlomo ad loc).
