A friend of mine referred an article to me, EXODUS 21:22-23: THE MISCARRIAGE INTERPRETATION AND THE PERSONHOOD OF THE FETUS, which made me reconsider my position on abortion. The main thesis of the article is that the differing punishment for different fetal development is due to differing legal statuses, and not personhood, by way of comparison to how the Old Testament and Ancient Near East cultures treat slaves and citizens differently.
To recapitulate: the traditional Catholic position was based on Exodus 21:22-23 which is accurately translated by Septuagint to make a distinction between "fully formed" fetuses, which accidental death attracts a life for life penalty, while not fully formed fetuses attracts a fine from the husband. The article contends that the "not fully formed fetuses" do not differ in personhood but in legal status by way of comparison to how slaves were treated differently from citizens. This can be seen directly in the same chapter in verses 28-32 where if an ox, which the owner knows is in the habit of goring and killing people and yet neglects to confine it, kills a person, the ox shall be stoned and the owner put to death. However, if the ox kills a slave, the owner need only compensate the master a monetary sum while the ox shall be stoned. Thus, while a slave obviously does not differ in personhood/humanity from a free Israelite, the punishment differs by virtue of their social status.
The same argument could be made by extension to fetuses, thus they do not differ in personhood or humanity but in legal status, the partially formed fetus can be compared to a slave, it is considered the property of the parents the same way a slave is the property of the master, but fully formed fetuses have full social standing, as it were, and the death of which will attract a life for life penalty.
A problem however remains. Even if we grant partially formed fetuses "full" personhood and humanity, there still remains the problem of the fact that they do have different legal status and as such attract differing legal punishments, would that not mitigate against applying similar penalties for partially formed fetuses and fully formed fetuses?
We could go down an extra-Biblical extrapolation to argue that just as the legal distinction between slaves and freeman should be abolished and recognise the equality of all, so should we abolish the legal distinction between fully formed and partially formed fetuses. But I am wary of appealing to such an extrabiblical egalitarian premise.
A different route would be to make a distinction between intentional homicide and unintentional homicide. The Exodus 21:22-23 passage concerning the partially formed and fully formed fetus is expressly to do with accidental death, i.e. when in the course of a struggle a man hits a pregnant woman and kills the fetus accidentally.
So the argument goes that in the case of intentional homicide, assuming a direct analogy between partially formed fetus and slaves, just as intentional homicide for slaves will attract a capital punishment, likewise will intentional homicide for partially fetuses attract a capital punishment. However, in my opinion, there is a step in the proof which is particularly weak. The premise that intentional homicide for slaves will attract a capital punishment hangs on verse 20 (which incidentally directly precedes the fetus passage): "If a man strikes his male or female slave with a rod and he dies at his hand, he shall be punished."
The article in question argues that "punished" here "traditionally" has been interpreted as "punished with death" or a capital punishment. I'm not sure if I am satisfied to go with such an appeal to tradition in the absence of much more explicit texts.
In sum here are my conclusions:
(1) I am thoroughly persuaded by the article's negative argument that differing punishments for fetuses in different stages of development has no effect whatsoever on their personhood or humanity by way of direct analogy to differing punishments between slaves and freeman within the very same chapter.
(2) I think viewing partially formed fetuses as property of their parents, analogous to slaves as property of their masters, is the correct matrix for understanding their legal status. This legal status obtains by virtue of their differing developmental stages, not by virtue of their prior history as being captured in war or having sold themselves into slavery. As such, it is a legal status which obtains by biology, not social practice.
(3) I am persuaded that intentional killing a partially formed fetus is not a tort and should be criminalised as an offense against a human life.
(4) My main concern now is simply whether or not that crime is a capital one or not. This in my opinion is the weakest link in the proof towards the idea that abortion of embryos should also attract a life for life penalty. I am however willing to contemplate the point that abortion of partially formed fetuses should at the minimum attract some sort of manslaughter charge contra Blackstone's argument that it is merely a misdemeanor.