My question goes back a few daf, in the sugyah where we say that we don't believe that a man did not have biah with a woman within 30 days, because Ein Adam Misapek 30 Yom. In ordinary circumstances at some point in those 30 days the woman will become nidah. If this happens toward the end of the 30 days, he will not be able to have biah and then the 30 days will go by without biah. Since this is so how can the gemarah apply the principle of Ein Adam Misapek 30 Yom?
Yehuda Gellman , Jerusalem
I would like to suggest the following answer to this very interesting question.
We are going to learn soon, b'Siyata di'Shmaya, at the beginning of Kesuvos (3b-4a), that the Gemara discusses the Halachah in the tragic circumstances that a parent of the Chasan or Kalah passed away just before the wedding. The Gemara (4a) states that if the bride became a Nidah before there had been Bi'ah, the couple is not allowed to sleep in the same room. The reason is that even though there generally is no Isur of being alone with one's wife when she is Nidah, this is because they previously had Bi'ah, but with a new wife before the first Bi'ah the temptation is very great, and therefore the prohibition of Yichud applies (this is the Halachah as recorded in Shulchan Aruch Even ha'Ezer 22:1).
Therefore, in your question, Reb Yehuda, we assume that she became a Nidah before Bi'ah took place. If so, they anyway must separate, and therefore the days when they are separate do not count towards the total of 30. When she goes to the Mikvah and becomes Tahor, we return to the count up to 30 days.
Kol Tuv,
Dovid Bloom
After thinking more about this question, and looking in the Mefarshim, I have a few thoughts to add.
In fact I have not found that anyone asks this question directly, which in itself is interesting, but I think that this omission can also be explained.
1) Firstly, let us look at the way the Shulchan Aruch cites the din of our Gemara. This is in Even Ha-Ezer 167:6. It simply states that if a Yevama claims in the first 30 days that there was no biyah, then the Halacha is that even though he says there was biyah and afterwards he divorced her, we still force him to do chalitzah. We pay attention to what she says that there was no biyah and this is why she requires chalitzah.
In the Shulchan Aruch there is no mention at all of the fact that in normal circumstances some of the 30 days are nidah days.
Again, I argue that if there were nidah days included in the figure of 30, then these would not count, because the couple would not be allowed to be alone when she is nidah, and she would be believed for up to 30 days that they were living together, even though together with the nidah days this would come to more than 30. The Shulchan Aruch does not mention the nidah days, because it is obvious that these are not considered a part of the 30, since they were not together then.
2) However there is another din in Shulchan Aruch which states that sometimes they may stay together even during the nidah days. This is in Yoreh Deah 192:4 in the Rema. This chapter in S.A. is about the bride who just got married. An opinion is cited there, that if she was tahor at the time they got married, but even so he did not have biyah with her and afterwards she became a nidah, in such circumstances there is no prohibition subsequently against yichud. Shach #10 writes that the fact that he did not have biyah when it was permitted, suggests that he is capable of controlling himself, and therefore it is not necessary for them to be separate during the nidah days.
3) In addition, the Ramban writes in our sugya in Yevamos 112a DH BeSheGitah, that in a scenario where she is forbidden to him, a man is believed to say there was no biyah even if 30 days had already gone by. Ramban writes that if there was an issur on her, he would be believed for his entire lifetime to say there was no biyah.
Therefore if we put the opinions of the Ramban and the aforementioned Rema together, we could have the following scenario, for instance. She had just become tahor when he brought her in for yibum and then 20 days went by and she became nidah. Since it might take another 12 days until she is able to go to Mikveh, it should emerge that she would be believed for 32 days that there had not been biyah.
4) I argue that it may be that in some rather unusual cicumstances such as the above, that she may receive more than 30 days of trust, or alternatively, it could be that we say that Chazal did not make any distinctions, and the maximum amount of days that she receives that she may be believed, is always no more than 30. However the above possibility is not so common, firstly because the Rema himself there writes that it is better to be stringent and not to rely on this leniency. In addition the Taz 192:7 disagrees with the Rema and writes that they are obliged by the Din to be separated, not merely as a chumra, and the Krati u'Plati (By Rav Yonatan Eibshitz) writes that the Rambam would also disagree with the din of the Rema and require them to be separated when she is Nidah.
5) In conclusion, I argue that the reason that the Poskim do not mention the role that the Nidah days play here, is because it is obvious that since they are living separately during these days, that they do not count towards the 30.
KOL TUV
Dovvid Bloom