Abortion: Augustine, in common with most other ecclesiastical writers of his period, vigorously condemned the practice of induced abortion. Procreation was one of the goods of marriage; abortion figured as a means, along with drugs which cause sterility, of frustrating this good. It lay along a continuum which included infanticide as an instance of "lustful cruelty" or "cruel lust" (nupt. et conc. 1.15.17). Augustine called the use of means to avoid the birth of a child an "evil work": a reference to either abortion or contraception or both (b. conjug. 5.5).
Augustine accepted the distinction between "formed" and "unformed" fetuses found in the Septuagint version of Exodus 21:22-23. While the Hebrew text provided for compensation in the case of a man striking a woman so as to cause a miscarriage, and for the penalty to be exacted if further harm were done, the Septuagint translated the word "harm" as "form," introducing a distinction between a "formed" and an "unformed" fetus. The mistranslation was rooted in an Aristotelian distinction between the fetus before and after its supposed "vivification" (at forty days for males, ninety days for females). According to the Septuagint, the miscarriage of an unvivified fetus were vivified, the punishment wa a capital one.
Augustine disapproved of the abortion of both the vivified and unvivified fetus, but distinguished between the two. The unvivified fetus died before it lived, while the vivified fetus died before it was born (nupt. et con. 1.15.17). In referring back to Exodus 21:22-23, he observed that the abortion of an unformed fetus was not considered murder, since it could not be said whether the soul was yet present (qu. 2.80).
The question of the resurrection of the fetus also exercised Augustine, and sheds some light on his views on abortion. Here again he referred to the distinction between the formed and unformed fetus. Though he acknowledged that it was possible that the unformed fetus might perish like a seed, it was also possible that, in the resurrection, God would supply all that was lacking in the unformed fetus, just as he would renew all that was defective in an adult. This notion, Augustine remarked, few would dare to deny, though few would venture to affirm it (ench. 33.85). At another point Augustine would neither affirm nor deny whether the aborted fetus would rise again, though if it should be excluded from the number of the dead, he did not see how it could be excluded from the resurrection (civ. Dei 22.13).
--from John C. Bauerschmidt, "Abortion", in Augustine Through The Ages: An Encyclopedia, edited by Fr. Alan Fitzgerald, OSA, p. 1.