Yevamot

that whosoever is subject to the obligation of levirate marriage is also subject to halizah, and whosoever is not subject to the obligation of the levirate marriage is not subject to halizah!  Rather, said Raba, it is this that was meant:  Where a levir married his yebamah who was found to be pregnant, her rival may not be married, because it is possible that the child would be viable, and marital contact with a pregnant woman is no proper marriage nor is the halizah of a pregnant woman proper halizah, while the child does not bring exemption until he is actually born. It was taught in agreement with the view of Raba: Where a levir married his yebamah who was found to be pregnant, her rival may not be married, because it is possible that the child would be viable, and neither marital contact nor halizah but only the child brings exemption; and the child brings exemption only after he is born. The reason, then,  is because it is possible that the child might be viable, but where the child is not viable her rival is exempt;  does this imply an objection against Resh Lakish?  — Resh Lakish can answer you [that the Baraitha] is thus to be interpreted:  Where a levir married his yebamah who was found to be pregnant, her rival may not be married; since it is possible that the child may not be viable, and the halizah of a pregnant woman is no valid halizah nor is the marital contact with a pregnant woman a proper marriage; and were you to suggest that one should be guided by the majority of women, and the majority of women bear healthy children, [it could be retorted that] a child brings no exemption until he is actually born. Said R. Eleazar: Is it possible that there should exist [such a ruling as] that of Resh Lakish and that we should not have learnt it in a Mishnah? When he went out he carefully considered the matter and found one. For we learned: If people came to a woman whose husband and rival had gone to a country beyond the sea and told her, 'Your husband is dead',  she may neither be married  nor be taken in levirate marriage  until she has ascertained whether her rival  is pregnant.  One can well understand why she may not be taken in levirate marriage, since it is possible that the child  may be viable and [the levir] would thus  infringe the Pentateuchal prohibition against [marrying] a brother's wife: but why should she not perform the halizah? It is possible to understand the reason why she must not perform the halizah within the nine months  and also contract a marriage within nine months,  since such [procedure would naturally be forbidden on account of the] doubt;  but let her perform the halizah within the nine months  and be married after the nine months!  — But even in accordance with your view,  let her perform the halizah and be married after the nine months!  The fact, however, is that nothing may be inferred from this;  for both Abaye b. Abin  and R. Hinena b. Abin  stated:  It is possible that the child  might be viable  and you would then subject her to the necessity of an announcement  in respect of the priesthood.  — Well, let her be subjected!  — It may happen that someone would be present at the halizah and not at the announcement,  and would form the opinion that a haluzah was permitted to a priest. Said Abaye to him: Was it stated, 'She shall neither perform halizah nor be taken in levirate marriage'? The statement, surely, was, 'She shall neither be married nor be taken in levirate marriage'  without halizah; if halizah, however, had been performed  she would indeed have been permitted! It was taught in agreement with Resh Lakish: Where a levir participated in the halizah with a pregnant woman who subsequently miscarried, she is required to perform halizah with the brothers. Raba said: The law is in accordance with the views of Resh !! If on the other hand, a viable child had been born, exemption took effect at his birth, and subsequent marriage would consequently be lawful. As the Mishnah, however, forbids halizah and marriage even after the nine months, unless definite information about the rival had been received, it must be assumed to represent the view of Resh Lakish who deems a halizah invalid wherever the child is not viable and the ceremony took place during pregnancy. Lakish in the following three rulings.  One is the ruling just spoken of. Another is his ruling in connection with the following Mishnah:  If a man  distributed his property verbally  and gave to one [son] more and to another less, or if he assigned to the firstborn a share equal to that of his brothers,  his arrangements are valid.  If, however, he said, 'As an inheritance',  his instructions are disregarded.  If he wrote  either at the beginning or the end or the middle, 'as a gift',  his instructions are valid.