The following discussion of this passage is offered by Prof. D. DeMarco
 The Roman Catholic Church and Abortion: An Historical Perspective - Part I [http://www.catholicculture.org/docs/doc_view.cfm?recnum=3361 ];

[...] an incorrect translation (intended or unintended) in the Septuagint version gives a totally different meaning to this Mosaic Law. The word zurah or surah, which means form, is erroneously used for the word ason, which means harm. Thus, the Septuagint version conveys the meaning of the fetus "not being further formed" rather than the woman "not being further harmed." The penalty, therefore, was now understood to be a fine if the fetus was not formed, but death if the fetus was formed. Thus, through a mistranslation by Hebrew scholars who were conversant with Greek thought, the distinction between the "formed" and "unformed" or "pre-formed" fetus was given moral significance and Biblical authority. Hebrew thought had never divided man into body and soul. The notion that the fetus could be unformed was more compatible with contemporary Greek thought, which had already believed that human life begins at some stage in fetal development when "ensoulment" or "animation" takes place. Aristotle had identified this time of animation with observable movement and believed it differed according to sex:

In the case of male children the first movement usually occurs on the right-hand side of the womb and about the fortieth day, but if the child be a female then on the left-hand side and about the ninetieth day.
[Aristotle, "History of Animals," The Works of Aristotle, Vol. II, Bk. 7, Ch. 3, 583b, p. 109. Felinus Sandaeus of Ferrara (d. 1503) calculated that animation took place on the fortieth day for the female and on the eightieth day for the male fetus.]

This erroneous statement of fact, with its curious numerical pinpointing of the time of animation for male and female, was to have a long life in biological and legal circles. The authority of Aristotle, which was based on his genius for observation and systematic thought, influenced the uncritical acceptance of this error.

The Septuagint mistranslation of the Exodus passage had allowed Greek thinking in biological matters to gain a theological respectability it did not deserve. Nonetheless, this thinking, involving the distinction between the preformed and formed fetus, provided the basis for a lively debate that continued for several centuries. In one sense the Septuagint text provides a strong argument against abortion by implying that killing a fetus already formed—which would exact the death penalty for the assailant—is equivalent to homicide. At the same time, it provides a basis for the claim that aborting a fetus not yet formed is neither immoral not unlawful.