Can an unborn infant be martyred?

[Geremia]

Pope Benedict XIV Doctrina de servorum Dei beatificatione et beatorum canonizatione lib. 3 cap. 15 "De requisitis in martyre necessariis ante martyrium":

Martyrium dicitur baptismus sanguinis Marc. 10. In adultis est actus fortitudinis, et caritatis : quoad infantes, sufficit occisos fuisse ex odio in Christum, ut martyrium suppleat vices baptismi aquæ, deleatque ex opere operato peccatum originale quoad culpam, et poenam, ut docet S. Thomas (2, 2, qu. 124, art. 1 ad 1)

However, he denies it:

Immo id ipsum extendunt ad infantes in uteris maternis occisos.

because someone never born once cannot be reborn in Christ (Jn. 3:3).

What constitutes killing out of hatred of Christ (ex odio in Christum) is not as restrictive as some may think. As Cdl. Billot, S.J. (De ecclesiæ sacramentis t. 1 p. 242) mentions, a "formal hatred of Christ and His religion" is not necessary, as some emperors who martyred did so only out of motives of ignorance of Christianity, upholding civil law, or public opinion; however, they did target Christians specifically.

Cdl. Cajetan, O.P., in his commentary on III q. 68 a. 11 ("Whether a child can be baptized while yet in its mother's womb?") is even more explicit, saying that even in utero infants can be martyrs like the Holy Innocents:

Si enim propter Christum infans in utero occideretur, martyr esset, non minus quam Innocentes.

The context is that "parvulos in maternis uteris periclitantes posse salvari" ("children in peril in the mother's womb can be saved"):

Posse autem salvari dico per sacramentum baptismi, non in re, sed in voto parentum susceptum, cum aliqua benedictione prolis seu oblatione ipsius ad Deum, cum invocatione Trinitatis.
[I say the child can be saved by the sacrament of baptism, not in re, but received in voto of the parents, with some blessing of the child or offering him to God, invoking the Trinity.]

^{(1917 Code of Canon law cans. 746-748 is along the same spirit.)}
In support of this, he gives two arguments:

• God's mercy provides for man in any state he may be in, and
• since they can be martyred (baptized by blood), it is reasonable that the parents' will can also bring them to the same end.

cf. these other sources indirectly related to this question: Pope St. Gregory the Great, St. Robert Bellarmine, and Vatican II preparatory commission

[Geremia]

De Sacramento Baptismi p. 166, first full ¶:

Some [who argue martyrdom alone cannot justify] respond that the Innocents were circumcised, hence justified before martyrdom. But this is not so. For it is uncertain whether they were all circumcised; nay, it is very probable and almost certain not all were circumcised. For Herod killed all the children who were in Bethlehem and its environs; he did not command only the children of Jews but utterly all be killed: and perhaps not a few Gentiles were among them. Moreover, he ordered that all infants two years and younger be killed, hence even those who were not yet 8 days old, who alone can be circumcised. Still, it is not de fide that circumcision justifies, and yet the Church certainly believes that absolutely all those infants are saved. Further, the Church not only believes the Innocents are saved but even honors them as martyrs; therefore, suffering for Christ conveys something to them ex opere operato. Nor is Scotus's argument that this is a privilege of children count: for it is asserted without any grounds. For if martyrdom profits infants ex opere operato, why not adults? Certainly the martyrdom of adults is no less powerful and efficacious than of infants; rather, it is more powerful, more noble, and more efficacious.