The mitzvah of yibum only applies if a man dies without having any children. The Mishnah (4:1) discusses a case where the brother, the yabam, performs chalitzah and soon discovers that the widow, the yavama, is pregnant. The Mishnah teaches that if she gives birth, and the child survives, then the chalitzah is meaningless since she did not require chalitzah. The Mishnah teaches that they would be able to marry each relatives and she would not be defined as a chalutzah and would be able to marry a kohen. If however she miscarries, then the opposite would be true.
The Gemara (35b) records the debate in case where she miscarries. R' Yochanan maintains, that since chalitzah has been performed, nothing further is required and she may now marry someone else. Reish Lakish however argues that chalitzah is still required. The Gemara explains that there are two ways to understand debate. Either they argue regarding the interpretation of a passuk or the debate is based on logic. According to the latter understanding, R' Yochanan argues that if we knew in advance that the pregnancy was not viable, then certainly chalitzah would work. Consequently, once she miscarries we now know that at the time of chalitzah the pregnancy was not viable and chalitzah should be effective. This logical tool is referred to as "tigli milta le'mafreah". Reish Lakish however argues that in this context we cannot say that information learnt latter can define an early status; so the earlier chalitzah is invalid. We shall try to understand the position of Reish Lakish.
The Ritvah cites his Rav, the Ra'ah, who explains that if at the time of chalitzah the child was not going to survive, then Reish Lakish would agree that the chalitzah is effective. His concern however is that at the time, the child was healthy, and there was a later incident that caused the miscarriage. Consequently, Reish Lakish requires chalitzah again, based on this doubt.
The Ritvah also cites the Tosfot who explain that according to Reish Lakish, chalitzah during any pregnancy is not valid. This is because he maintains that the Torah only required chalitzah when there is no child (viable or otherwise) in the world from the couple. Note that this explanation is not based on a doubt, but rather the chalitzah is definitely ineffective. Importantly, this understanding is that chalitza is ineffective and not that yibum or chalitza is not required. For if she miscarries, then Reish Lakish still requires chalitza. If she were truly exempt, then this would continue to be the case.
The Tosfot printed in our Gemara however explains that according to Reish Lakish, tigli milta le'mafreah is only useful when the law now is dependant on the current status which may be revealed at a latter point. This case however is different, in that we want to know now what the status will be in the future. Consequently, it does not help when that status is learnt.
The Tosfot ask a further question, that if explored will help us to understand our debate. The next Mishnah teaches that if he performed yibum, and then she miscarries, he is not required to bring a korban for having a forbidden relationship. The Tosfot however ask, that according to Reish Lakish, the relationship he had while the yavama was pregnant should not be defined as yibum (much like chalitzah at that time is meaningless). That being the case, it should be considered as if he had a relationship with his brother's (ex-)wife, not in the context of yibum which is prohibited and he should be required to bring a korban. The Tosfot however answer by citing another case, were a katan tried to perform yibum, where even though the mitzvah has not been performed, no prohibition has been violated. Consequently, that would be that case in this situation as well. How do we understand this answer?
The Ritva explain in a similar way to the Tosfot cited by the Ritva above. He explains that it is true that bi'ah (the relationship) did not qualify as yibum at that time – he terms it bi'ah geru'ah. However since once she miscarries she still requires yibum (she is zekuka) it is not considered as though the bi'ah was not in the context of the mitzvah of yibum. He asserts that if it were truly prohibited due to her being his brother's (ex-)wife, then since she was prohibited at that moment, then even after the miscarriage yibum would have been prohibited.
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