One class of arguments which is sometimes seen from pro-choice advocates relies on the fact that a large number of embryos die very early in pregnancy. From this they extrapolate that inducing abortion is permissible. Henry Morgentaler, the well-known Canadian abortion practitioner, makes this argument (‘The Moral Argument for Abortion’, Free Inquiry 16, 3 (1996), p. 18):
If abortion is always viewed as “intentional murder,” why isn’t miscarriage viewed in similar terms? After all, almost half of all embryos are spontaneously shed in what is called “miscarriage” or “spontaneous abortion.” If spontaneous abortions are an “act of God,” to use the common religious expression, is it not strange that God has so little concern for fetal life that He allows so much of it to go to waste without intervening? Is it not possible to then conclude that God does not mind or object to spontaneous abortions? Why is it that the Catholic church has no ritual to mark the abortion of so much fetal life when it occurs spontaneously, yet becomes so vociferous and condemnatory when it is a conscious decision by a woman or couple?
A more recent example was made by Bill Nye in a BigThink video, which was panned in the National Review by two Catholic ethicists, Robert P. George and Patrick Lee. What is interesting about this is that arguments in the same class have been made by Catholic theologians in the past; not, of course, to show the permissibility of abortion, but to argue against immediate hominization. Karl Rahner, for example, wrote:
For a few centuries Catholic moral theology has been convinced that individual hominization occurs at the moment of the fusion of the gametes. Will the moral theologian still have today the courage to maintain this presupposition of many of his moral theological statements, when he is suddenly told that, from the start, 50% of the fecundated female ova never reach nidification [i.e., implantation] in the uterus? Will he be able to admit that 50% of the “human beings”—real human beings with an “immortal” soul and an eternal destiny—do not, from the very start, get beyond this first stage of a human existence? (Cited in Joseph F. Donceel, ‘Immediate Animation and Delayed Hominization‘, pp. 99-100).
What is more, Anselm of Canterbury makes a similar-sounding argument:
Anselm wrote that it is inadmissible that the infant should receive a rational soul from the moment of conception. This would imply that every time an embryo perishes soon after conception, a human soul would be damned forever, since it cannot be reconciled with Christ, “quod est nimis absurdum.” (From p. 78 of Donceel, ‘Immediate Animation and Delayed Hominization’. He sources the quote to “Liber de conceptu virginali et originali peccato 7 (PL 158, 440)”).
There is, however, a difference between the arguments made by the modern authors and the one made by Anselm. According to Rahner, for example, what is absurd is that God would allow so many people to die at the very first stage of existence. Anselm, meanwhile, argues that it is absurd that God would allow so many people to die without possibility of baptism. This I think highlights a flaw in the argument that many moderns miss, which is that for most of human history, infant mortality has also been extremely high, at times rivaling even the figure of 30%-50% of embryos which are said to die very early in pregnancy (even today, in certain countries the infant mortality ratio is 10%). It would of course be ridiculous for ancients to argue that since so many infants die, perhaps infants are not yet people. It would be even more absurd if they used this to argue for the permissibility of infanticide. But since Anselm’s position is about salvation more than it is about death, no such reductio objection is possible, as infants can be baptized, and so the high mortality of infants (with which Anselm was presumably aware) poses no problem for his argument.
While Latin theologians, following Aristotle, typically favored delayed hominization, Eastern theologians, including the Cappadocians and Maximus the Confessor, have typically favored immediate hominization upon conception. I’ve had to rely pretty heavily on secondary sources for what Catholic theologians have historically made of the high incidence of miscarriage, so what I can present is pretty limited. It would be interesting to see how much ‘air-time’ arguments in this class got among ancients or medievals outside of Anselm.