וְכִֽי־ יִנָּצ֣וּ אֲנָשִׁ֗ים וְנָ֨גְפ֜וּ אִשָּׁ֤ה הָרָה֙ וְיָצְא֣וּ יְלָדֶ֔יהָ וְלֹ֥א יִהְיֶ֖ה אָסֹ֑ון עָנֹ֣ושׁ יֵעָנֵ֗שׁ כַּֽאֲשֶׁ֨ר יָשִׁ֤ית עָלָיו֙ בַּ֣עַל הָֽאִשָּׁ֔ה וְנָתַ֖ן בִּפְלִלִֽים׃
אִם־ אָסֹ֖ון יִהְיֶ֑ה וְנָתַתָּ֥ה נֶ֖פֶשׁ תַּ֥חַת נָֽפֶשׁ׃
As we shall see this is a notoriously difficult passage to translate, and each translation carries very significant life and death ramifications. For now, let's stipulate that the passage means:
If men fight and hit a pregnant woman, and her offspring comes out, but there is no damage, he [the one who hits her] will be fined whatever her husband imposes, and the judges approve. But if there is damage, you shall give life for life...
The verse begins with a case that caused "no damage." So, what happened to the offspring? At first glance, it may seem that both mother and child are fine. However, no ancient interpreter read the passage this way, reasoning that if neither mother nor child had suffered an injury, no fine could be imposed. The fact that the Torah allows the father to collect a penalty from the man who stuck his wife was taken as solid proof that the passage is speaking of a case where either the mother or the child was harmed.
And, indeed, this is how the LXX's translator read the verse:
And if two men strive and smite a woman with child, and her child be born imperfectly formed, he shall be forced to pay a penalty: as the woman’s husband may lay upon him, he shall pay with a valuation. But if it is perfectly formed, he shall give life for life [translation]
The implication of this translation is that if the accident-causing miscarriage occurs when the child is still imperfectly formed the offender isn't guilty of murder, and he's subject to a fine, not capital punishment. Though no definition is given for perfectly or imperfectly formed, it follows that the author of the LXX translation would say that a late abortion is considered murder, but an abortion carried out before the fetus is whatever is meant by "fully formed" subjects the abortionist to a fine, and he is not considered a murderer.
Jerome, author of the Vulgate translation which became the official bible of Roman Catholicism, read the verse differently:
If men were fighting and someone struck a pregnant woman and she miscarried but she herself lived, he will be subject to a fine, as much as the woman's husband shall request and as the judges decree. If however, her death shall follow, let him pay a soul for a soul. [translation is Kugel's]
Jerome, it seems, was of the opinion that no matter when the fetus dies - first month or last - only a fine is imposed on the one who killed it. As he reads the verse, the word "damage" refers to the death of the mother - if she lives, a fine is imposed to cover the death of the fetus, but if she dies a murder has been committed and capital punishment if imposed. As James Kugel points out, Jerome could have only read the verse this way if he believed that a fetus is not a separate human being. His translation seems based on the premise that a mother and her fetus are one entity so long as the fetus remains inside its mother. This is why the death of the fetus carried only a fine, a penalty identical to the one imposed for any other non-fatal injury.
And how did the ancient Rabbis (Chazal) read this verse? Ironically enough, they seem to have taken the same approach as Jerome. In many places they uphold the principle ubar yerech imo, a fetus is a limb of its mother. They rule that someone who causes a miscarriage is subject only to a fine. Along with this, the consequences of the ubar yerech imo principle include the following laws (some of which have additional justification aside from ubar yerech imo):
(1) If a cow is disqualified for food or sacrifice, the fetus inside her is also disqualified.
(2) A fully-formed fetus found inside a properly slaughtered animal does not require a ritual slaughter of its own
(3) A pregnant animal may be brought as a sacrifice
(4) If a pregnant woman is sentenced to death, we don't wait for her child to be born before carrying out the execution
(5) If a pregnant woman's life is threatened by her fetus, we kill the fetus to save the mother.
Though the ancient Rabbis follow interpretation #2 (ie, Jerome's approach) of Exodus 21:22, modern Jews do not allow allow abortion under all circumstances (as a straight reading of Jerome seems to justify) and neither do Roman Catholics. Modern Catholics and modern Rabbinic Jews have come to different conclusions about the legality of abortion based, in part, on other verses. It is not known what the ancient rabbis thought about the matter, as abortion was uncommon. (see postscript #2).
PostScript 1: Kugel notes that we do know of one community of Jews that did not seem to accept ubar yerech imo. The Dead Sea Scroll community followed interpretation #1 (ie the LXX approach) and outlawed bringing a pregnant animal for a sacrifice.* They also considered the mother of a still born child to be ritually impure (because she came in contact with a dead body, and not a "limb" of her own.) I don't know what the DSS Jews did in the case of a fetus that threatened the life of its mother, though in the ancient world it must have come up regularly: Before c-sections and labor- inducing techniques it was not uncommon for a drawn-out delivery to threaten the mother's life. In such a case, both Jerome and the Rabbis permitted killing the fetus - even at the very end of the ninth month - to save the mother. Did the DSS Jews allow this, too, or did they let the mother die?
Postscript 2: Though the Rabbis allowed the fetus to be killed in a case of portracted labor that threatened a mother, the justification beneath this ruling is unclear. Was it because of the principle of ubar yerech imo? Was it based on their interpretation of Exodus 21:22, which did agree with Jerome, and did conclude that causing a miscarriage only resulted in a fine because the unborn child was still part of his mother? Or did they rely on a different principle, the din rodef, which says we can kill a person who is on his way to kill another person? I confess to being confused. I know the Rambam and others who wrote later allowed the fetus to be killed based on rodef, however its not clear to me that Chazal embraced this justification alone, as they did believe ubar yerech imo and they did translate Exodus 21:22 accordingly.
* There's a verse somewhere in Leviticus that says you can't kill a mother animal and its offspring on the same day. DSS Jews included the fetus in this, based on how they understood Exodus 21:22. Rabbinic Jews excluded the fetus from the Leviticus law.