As we have learned on yesterday’s daf, if a widow is pregnant when her husband passes away, there will be no mitzva of yibum if she gives birth to a viable child. If the child dies, however, and the dead man had no other children, the normal rules of yibum would apply.
How long does a child have to live in order to be considered “viable”?
Rabbi Shimon ben Gamliel teaches that once a child lives for 30 days he is considered a live birth. This teaching implies that a child who dies within the first 30 days after birth is, at best, a safek – that his status is questionable.
The Gemara presents a case where a woman in this situation, i.e. whose child died before 30 days had passed, married someone from outside the family (that is to say, she assumed that she was not obligated in yibum, since she had a child – albeit one who did not live for very long – with her first husband). In such a case, Ravina quotes Rava as ruling that she should have halitza done if she married a regular person, but if she married a kohen – who is not permitted to marry someone who had undergone halitza – then halitza should not be done, since it would destroy the woman’s marriage. Rav Mesharshiya, however, quotes Rava as saying that even if she married a kohen, halitza is necessary. Ravina tells Rav Mesharshiya that although Rava had said that in the evening, by the morning he had changed his mind. In response, Rav Mesharshiya says sarcastically “You say it is permitted? Then you can permit forbidden fats (heilev) as well!”
The Tosefot HaRosh says that Rav Mesharshiya was arguing that permitting a woman who had had halitza to a kohen would lead to other mistaken rulings – like permitting a divorced woman to marry a kohen, as well. The Arukh LaNer suggests that Rav Mesharshiya was hinting to a Gemara in Massekhet Hullin (49a) that kohanim were lenient and permitted certain questionable fats. Thus, Rav Mesharshiya is arguing that someone who is so desirous of helping out kohanim should rule leniently on the subject of heilev.