19 You ask what was the manner in which, as the Spirit teaches, the Son was born? I will put a question to you as to things corporal. I ask not in what manner He was born of a virgin; I ask only whether her flesh, in the course of bringing His flesh to readiness for birth, suffered any loss. Assuredly she did not conceive Him in the common way, or suffer the shame of human intercourse, in order to bear Him: yet she bore Him, complete in His human Body, without loss of her own completeness. Surely piety requires that we should regard as possible with God a thing which we see became possible through his power in the case of a human being.
Source: On the Trinity (New Advent)