Homily 13 on the Song of Songs — as quoted by Pitre (pp 132, 135)
Among the myriads of men born of Adam, succeeding him as long as his nature will continue through successive births, only [Jesus] came to light through a new way of being born…In fact, his birth alone occurred without labor pains, and he alone began to exist without sexual relations…Even the prophet Isaiah affirms that her giving birth was without pain, when he says: 'Before the pangs of birth arrived, a male child came forth and was born' (Isa 66:7).
This happened for an understandable reason; there is nothing absurd about it. Just as she who introduced death into nature by sin was condemned to bear children in suffering and travail (Gen 3:16), it was necessary that the Mother of life, after having conceived in joy, should give birth in joy as well.
No freely usable English translation of this passage is online, so the note keeps Pitre's quotation and links to the full text. Translation: as quoted by Pitre (via Gambero); a modern translation by Richard Norris (SBL, 2012) exists.
In Jewish Roots of Mary
Why it is cited
Jesus' 'birth alone occurred without labor pains,' as Isaiah 66:7 affirms; and since Eve was condemned to bear in travail, 'the Mother of life, after having conceived in joy, should give birth in joy as well.'