Commentary for Yevamot 4:2

וחייבין בקרבן – for he had intercourse with his brother’s wife not in the place of the Mitzvah, and the offspring is kosher whichever way you turn.

Introduction Our mishnah deals with a case where the yevamah is found to be pregnant after having yibbum. In the previous mishnah she was found pregnant after halitzah.

וחייבין אשם תלוי – for everything where people are liable for extirpation for something done willfully, and for something done inadvertently for sin-offering, ae liable for something whether he became conscious [of his transgression] or not, a guilt offering is made when one is in doubt as to the commission of a sinful act (i.e., suspensive guilt-offering).

A yavam marries his yevamah and she is subsequently found to be pregnant, and she gives birth:
If the child is viable, he must divorce her and both are obligated to bring a sacrifice; If the child is viable, then the yibbum was unnecessary, for only the wives of childless husbands must go through yibbum. Not only was the yibbum unnecessary, but the marriage of the yavam to the yevamah was prohibited, for she is the wife of his brother [Marrying one’s brother’s wife is forbidden in all cases except for when yibbum is obligatory]. Therefore, he must divorce her and both must bring a sin-offering for having forbidden relations.

If the child is not viable, he may retain her [as a wife]. If the child was not viable, then the yibbum was valid and he may retain her as a wife. Neither must bring a sin-offering.

If it is doubtful whether it is a nine-months child of the first [husband] or a seven-months child of the second [husband] he must divorce her, and the child is legitimate, and they must bring an asham talui. If it is unclear whether the child was from the first husband, and the pregnancy was full-term, or the child was from the second husband (the yavam) and the pregnancy was only seven months long, then the yavam still must divorce her, lest the yibbum was unnecessary and she is forbidden to him because she is his brother’s wife. The child is not in any way illegitimate, whether or not it was from the first or second husband. Both the man and the woman must bring a sacrifice known as an “asham talui” (a dependent guilt-offering), which is brought when a person does not know whether or not what he did was sinful. In this case he has only transgressed if the child was not his. Note that the mishnah does not talk about an eight-month child. In the ancient world, it was considered impossible for a child born in the eighth month of pregnancy to live. Children born at nine or at seven months could live, but not those born after eight months. The rabbis also believed this “scientific” belief, and hence will often call a child born in the eighth month a “miscarriage”. If the child nevertheless lived, they probably assumed that they had counted the months wrong. The rabbis also developed a theory that there were two different types of pregnancies: a seven month and a nine month. If the child was born at eight months and lived, it must have been a seven month pregnancy that was overdue. If the child was born at eight months and died, it must have been a premature nine month pregnancy.