Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Councils of Ancyra, Gangra, Neocæsarea, Antioch and Laodicea, which Canons were Accepted and Received by the Ecumenical Synods.: Canon VI

p. 82 Canon VI.

Concerning a woman with child, it is determined that she ought to be baptized whensoever she will; for in this the woman communicates nothing to the child, since the bringing forward to profession is evidently the individual [privilege] of every single person.

Notes.

Ancient Epitome of Canon VI.

If a woman with child so desires, let her be baptized.  For the choice of each one is judged of.

Van Espen.

That the reason of the canon may be understood it must be noted that in the first ages of the Church catechumens were examined concerning their faith before they were baptized, and were made publicly to confess their faith and to renounce openly the pomps of the world, as Albaspinæus (Aubespine) observes on this canon, “A short while before they were immersed they declared with a loud voice that they desired baptism and wished to be baptized.  And since these confessions could not be made by those still shut up in their parent’s womb, to them the thing (res) and grace of baptism could not come nor penetrate.”  And altogether in accord with this is the translation of Isidore— “because the free will of each one is declared in that confession,” that is, in that confession he declares that he willingly desires to be baptized.