How long must a baby live before it is not deemed to be stillborn? This question is raised by the Gemara regarding the laws of circumcision, but is applied to other areas of Jewish law, as well.
It was stated that the Sages discussed the following question: What is the ruling in a case where a baby died within thirty days after birth, leaving its mother a childless widow, and before they decided whether or not she was obligated in levirate marriage, she stood and was betrothed to another?
Ravina said in the name of Rava: If she is the wife of an Israelite, meaning she became betrothed to an Israelite, who may marry a woman who has undergone ḥalitza, she performs ḥalitza due to uncertainty. Given that the child may have been stillborn and therefore never considered alive, in which case she would be obligated to undergo levirate marriage or performḥalitza, by performing% halitza, she removes any doubt and can remain with her new husband. However, if she is the wife of a priest, she does not perform %halitza, as if she were to perform ḥalitza she would be prohibited to her husband the priest. Since there are those who hold that the baby is considered alive from the moment of its birth, based on that opinion, she is exempt from performing ḥalitza, after the fact.
Rav Sherevya said in the name of Rava: Both this, the woman married to an Israelite, and that, the woman married to a priest, perform ḥalitza, as the prohibition against marrying a woman not released from her bond of levirate marriage is a stringent one, and the fact that her husband is a priest is not taken into consideration.
The laws of levirate marriage and ḥalitza appear in the Torah (Devarim 25:5–10) and are articulated at length in tractate Yevamot, which is devoted to this topic. The basic concept is that if a man dies with no living descendants, his wife is obligated by a levirate bond to her husband’s brother. The brother must either take her as a wife through levirate marriage, or sever the bond by means of the ceremony of ḥalitza. As long as the levirate bond is intact, it is prohibited for her to marry a different man, and if she does so, as long as her marriage continues she is performing an ongoing transgression. Although the halakhot of levirate marriage and ḥalitza are not like those of a regular marriage, and a woman who has undergone ḥalitza is not prohibited to marry a priest by Torah law, the Sages decreed that the legal status of a woman who undergoes %halitza is that of a divorcée, in the sense that it is prohibited for a priest to marry her.