The Idea in Brief
The LXX speaks of two outcomes: one regarding the developed fetus and one undeveloped. The Masoretic Text however simply presents the fetuses in the plural, and in this respect there is no amplification or clarification between developed and undeveloped in the Masoretic Text.
In summary, if the Masoretic Text carries the original literary form, then the LXX would be the wider explanation for the plurality of the fetuses through the two scenarios: one developed fetus and one undeveloped. However, if the plural form refers to an indefinite idea (according to recognized Hebrew scholars), then the answers appear to lead to something else: that is, there is distinct nuance between murder and manslaughter, which Jewish oral tradition makes clear.
The Samaritan Pentateuch and the LXX are contemporaneous witnesses, since both appeared several centuries before the current or common era. The Samaritan Pentateuch, like its LXX counterpart, addresses one fetus.
Exodus 21:22-23 (Samaritan Pentateuch)
22 If men strive, and hurt a woman with child, so that her fruit depart [from her], and yet no mischief follow: he shall be surely punished, according as the woman's husband will lay upon him; and he shall pay as the judges [determine]. 23 And if [any] mischief follow, then thou shalt give life for life.
According to Page 93 of the Vetus Testamentum Hebraicum cum Variis Lectionibus by Benjamin Kennicott, the verses above from the Samaritan Pentateuch are identical to the Masoretic text, except that the fetus here is in the singular and not plural (which is the case of the Masoretic Text).
To recap, the Samaritan Pentateuch provides its translation in reference to one single fetus. Its contemporary, the LXX, also provides for one single fetus, however, also provides for two scenarios regarding the one fetus. In other words, the original proto-Hebrew texts may have referred to a plurality of fetuses and therefore appeared ambiguous.
For example, another witness is the Targum Onqelos, which may have appeared as early as the third century of the current or common era. This Targum provides the plurality of fetuses, but without any amplification or explanation. In this regard, the Targum Onqelos is consistent with the Masoretic Text, which appeared many centuries later (and which also refers to the plurality of fetuses). The proposed translation of this passage from this Targum is as follows:
Exodus 21:22-23 (Targum Onqelos)
22 On account that men fight and strike the woman who is pregnant, and the offspring (lit., unborn children) come forth, and no death results, a claim will be demanded just as what is appropriate to him, the woman's husband, and they will pay according to the words of the judges. 23 And if there may be death, then they shall give life for life.
In summary, one hypothesis for these disparities between these various texts is that the proto-Hebrew texts were ambiguous, and the LXX was one attempt to amplify and clarify the plurality of fetuses in the passage. (The Samaritan Pentateuch eliminated the ambiguity and referred to only one fetus.) The Targum Onqelos, and later the Masoretic Text, preserved the original proto-Hebrew form with its ambiguity.
Why then the plurality of fetuses in the womb? The plural form was to express an indefinite idea in Biblical Hebrew (and therefore would be inclusive of all scenarios). For example, Keil & Delitzsch write the following. Please click on the image to enlarge.
In this regard, Gesenius also notes the following:
In Biblical Hebrew, the indefinite plural refers to the wider idea, and therefore would capture all nuances and scenarios. And so we are back at "square one." What then is the meaning of this passage, if the student relies on the plain and normal reading of Scripture? In this regard, the rabbis who preserved the oral traditions of the Jews provide the most sensible explanation: the idea in this passage concerns the difference between manslaughter, which is intentional, and murder, which is intentional.
In this regard, then, the Babylonian Talmud in Sanhedrin, Folios 78B, 79A, and 79B provides the most comprehensive and authoritative view concerning the passage in question from the perspective of Jewish oral tradition. (The hyperlink to the Talmud reference includes commentary in the last paragraph by the translator, Jacob Neusner.) That is, the rabbis understood from the Biblical passages that there were nuances between manslaughter and murder. In this regard, the intent to kill is what discriminated manslaughter from murder. In other words, the life of the mother and the life of the fetus have equal par in this passage, because in the event of either the death of the mother or of the death of the fetus, the result would have been manslaughter (since the men who were fighting had no intent of killing either one or the both of them). The passage therefore does not equate the life of the fetus to the level of common chattel, but to the same level of manslaughter as if the life of the mother too were killed (since both mother and fetus appear together in this passage as innocent bystanders). The point here is: is there deliberate intent to kill?
The following graph provides the visualization of the rabbinic logic as found in Sanhedrin concerning Exod. 21:22-25. Please click on the image to enlarge.
If the proto-Hebrew texts were ambiguous, then both the LXX and the Samaritan Pentateuch appear to deal with this literal ambiguity of the plurality of fetuses in the passage. In the LXX, the translation provides for two scenarios, which discriminate between the developed and the undeveloped fetus. In the Samaritan Pentateuch, the translation eliminates the ambiguity, and translates the meaning to refer to one fetus.
The Targum Onqelos and the Masoretic Text, however, preserve the ambiguity. The ambiguity, according to Jewish oral tradition, has to do with the nuances between manslaughter, which is unintentional, and murder, which is intentional. For example, in the case of murder, financial remuneration is not allowable (Numbers 35:31); however, in the case of manslaughter, when the "cool off" period in the cities of refuge has passed, the accused is not prevented from returning home (Numbers 35:25-28). In this regard, the passage at hand (Exod 21:22-25) allows for the accused to make restitution and pay civil damages (whether the mother and/or fetus died), because the accused did not commit murder, but had harmed them and/or committed manslaughter (since there never was any original intent to kill either one or both of them).
Gesenius, H.F.W. (1922). Gesenius' Hebrew Grammar (2nd Ed.). Oxford: Clarendon Press, 400.
Keil, C. F., & Delitzsch, F. (2006). Commentary on the Old Testament (Vol. 1). Peabody: Hendrickson, 409.
Neusner, Jacob (2007). The Babylonian Talmud: A Translation and Commentary (Vol. 16). Peabody: Hendrickson, 416-418.