...pueri in maternis uteris existentes nondum prodierunt in lucem, ut cum aliis hominibus vitam ducant. Unde non possunt subiici actioni humanae, ut per eorum ministerium sacramenta recipiant ad salutem. Possunt tamen subiici operationi Dei, apud quem vivunt, ut quodam privilegio gratiae sanctificationem consequantur, sicut patet de sanctificatis in utero.Aquinas [ST 3.68.11 ad 1, my translation; Dominican Fathers translation here]
Children in the maternal womb have not yet been born so as to lead a life with other human beings. Thus they are not able to be subject to human action so as by such ministry to receive the sacrament [of baptism] unto salvation. They are able however, to be subject to the action of God, in Whose presence they live, so as to attain by a sort of privilege sanctifying grace, as is obvious with those sanctified in the womb.
A privilege is a law that concerns itself with a single person; the examples of those sanctified in the womb that Aquinas has in mind would be the Virgin Mary, the prophet Jeremiah, and John the Baptist.