30 I will ask this objector in reply, whether he remembers my calling Him anything else than born, and whether I did not say that existence before times eternal and birth have the same meaning in the case of Him that was. For the birth of One already existing is not really birth, but a self-wrought change through birth, and the eternal existence of One Who is born means that in His birth He is prior to any conception of time, and that there is no room for the mind to suppose that at any time He was unborn. And so an eternal birth before times eternal is not the same as existence before being born. But to have been born always before times eternal excludes the possibility of having had no existence before birth.
Source: On the Trinity (New Advent)