Recently I came across the odd claim that Aquinas held that the fetus was "quickened" at 17 weeks. This is not an area at which I've looked too closely, but it's pretty clearly a garbling -- Aquinas always seems to get garbled on this subject. The seventeen weeks had nothing to do with Aquinas; it was established by a decree by Gregory XIV in 1591, and even then it was solely a matter of legal cut-off for penalties. That is, abortions prior to the 116 day mark, although still considered wrong, were not general grounds for excommunication. I'm not quite sure what the reasoning behind the number was. This 116-day line was removed by Pius IX, on the grounds (I think) that, biologically speaking, the cut-off was purely arbitrary in every way.
Aquinas in a very early text (in the Commentary on the Sentences lib. 3, dist. 3, art. 2) on whether Christ's conception was instantaneous mentions that Aristotle held that the process of conception took up to forty days for men and ninety days for women, briefly and without any comment except to note that Augustine thought the process was 46 days for the conception of men. As far as I'm aware, it's the only mention ever in Aquinas, and it's unclear how it relates to anything else; it's possible to read it as just the claim that the male body is visibly articulated as male at forty days and the female body at ninety (which is what Aristotle says miscarriages show). Quickening becomes a focus much later than Aquinas, so it's also unclear what he would think of it; since it only has to do with the spontaneous physical motion of the fetus, it has no intrinsic connection to whether the fetus has a rational soul (which Aquinas does think happens after an extended process of development during which the embryo is first sub-animal and then nonrational animal, but for which he gives, as far as I am aware, no timelines, unless the single 40/90 passage is counted). And, indeed, outside of common law, quickening completely lacks the importance often attributed to it until (as far as I can tell) the late nineteenth century; as far as I am aware, it was never before that time an element in moral discussions, nor in discussions about the origination of personhood.
