The foundational pasuk (Shemot 21:22–25) distinguishes between causing a miscarriage — which incurs only a monetary fine — and killing the mother outright, which incurs the full 'life for life' penalty; this textual asymmetry has anchored the rabbinic discussion of the fetus's status ever since, with the Gemara in Sanhedrin 72b drawing the further distinction that a fetus endangering its mother may be destroyed as a pursuer, but once the head has emerged one life may not be set aside for another, since at that point the danger comes from Heaven rather than from the child.
The Rambam (Mishneh Torah, Rotzeach 1:9) codifies this ruling explicitly, permitting the fetus to be cut away — whether by drug or by hand — when a woman is in mortal danger during labor, precisely because the fetus is treated as a pursuer, yet he equally forbids any intervention once the head has appeared.
The Gemara in Yevamot 69b describes the fetus within the first forty days as 'mere water,' and the Mekhilta (Mekhilta DeRabbi Yishmael, Nezikin 4) holds that one who kills a non-viable fetus incurs no capital liability, while Niddah 44a treats the fetus as a distinct body — together these sources establish that the fetus occupies a status below that of a fully born person, yet is not legally insignificant, a tension that underlies all later debate.
The Minchat Chinuch (34) notes that one who kills a fetus in the womb is exempt from capital punishment, yet raises the possibility that a fetus whose mother has died may have full legal status — illustrating that the halachic standing of the fetus remains genuinely unsettled at the margins.
Noda BiYehudah II (Choshen Mishpat 59) engages the question of when the prohibition against touching the emerging head applies, tying the ruling to whether the infant's months of gestation are known to be complete, and thereby sharpening the conditions under which the permissive ruling of the Mishnah operates.
Tosafot on Sanhedrin 72b is not available in full text for Mekoros to read, but our research indicates that he addresses the categorization of fetal killing and the pursuer principle as they apply here.
Iggerot Moshe (Choshen Mishpat 2:69) concludes, on our reading of the cited page, that destroying a fetus constitutes an actual prohibition of murder — applying equally to Jews and non-Jews — grounding this position in Tosafot in Sanhedrin, and thus represents the stringent pole of modern responsa, permitting abortion only in the narrowest of circumstances.
By contrast, Tzitz Eliezer (13:102) concludes, on our reading of the cited page, that terminating a pregnancy up to the seventh month is permitted when the fetus is diagnosed with a fatal disease such as Tay-Sachs, reasoning that the severe and certain emotional anguish of the parents constitutes a 'great need' legally equivalent to physical distress — representing a significantly more permissive position that nonetheless requires genuine, grave cause and operates within defined parameters.