Chapter 5 — Part I: The Creed — Article III: Who Was Conceived by the Holy Ghost, Born of the Virgin Mary
Section 8. In The Incarnation Some Things Were Natural, Others Supernatural — In this mystery we perceive that some things were done which transcend the order of nature, some by the power of nature. Thus, in believing that the body of Christ was formed from the most pure blood of His Virgin Mother we acknowledge the operation of human nature, this being a law common to the formation of all human bodies, that they should be formed from the blood of the mother.