פירוש על סנהדרין 144:3

its head came out: With a women that is experiencing difficulty giving birth and is in [mortal] danger. And it is taught in the first section [of this teaching], "the midwife extends her hand and cuts it up and extracts [the pieces];" as the entire time that that it has not gone out into the air of the world, it is not [considered] a soul, and [so] it is possible to kill it and to save its mother. But when its head came out, we cannot touch it to kill it, as it is like a born [baby]; and we do not push off one soul for the sake of another. And if you will ask [from] the story of Sheva ben Bichri - [wherein it is written] (II Samuel 20:21), "behold, his head is sent to you" - they pushed off one life for the sake of another; there, it was because even if they had not delivered him, he would have been killed in the city when Yoav would have seized it, and they would have been killed with him. But if he would have [otherwise] been saved - even though they would have been killed - they would not have been allowed to deliver him [to Yoav] in order to save themselves. And also (another answer) is that it is because he was a rebel to the kingdom, and so is it explained in the Tosefta (of Terumah).