I forgot to say that the above quote from the Catechism of the Council of Trent (aka the Roman Catechism) was from “The Creed,” Article III.
Mary’s lack of pain in childbirth is not talked about a lot, because it’s a delicate subject that’s not easy to bring up in a homily at Mass. (“Mommy, what does he mean, physical virginity? What’s a labor pain?”) But it has been a constant belief that has been constantly taught by the Church.
The miraculous birth of Christ shows dramatically that He was God as well as Man at all times, not just gaining His powers and omniscience later in life. (Maybe this is another reason that moderns don’t like His virgin and painless birth?) His Nativity also foreshadowed His emergence from the tomb, and His miraculous appearance in the Upper Room while the doors were all locked.
Here’s another official quote, a prayer from the Votive Mass of “Mary at the Foot of the Cross.” As they say, Lex orandi, lex credendi – the law of prayer is the law of belief:
“In your divine wisdom, you planned the redemption of the human race, and decreed that the new Eve should stand by the cross of the new Adam: as she became his mother by the power of the Holy Spirit, so, by a new gift of your love, she was to be a partner in his Passion, and she who had given him birth without the pains of childbirth was to endure the greatest of pains in bringing forth to new life the family of your Church.”
There’s a nice Middle English poem with Jesus speaking to Mary at the foot of the Cross, that puts it rather neatly:
“Moder, rewe of modres care
Now thou wost of modres fare,
Thogh thou be clene mayden-man.”
(“Mother, take pity on mothers’ care,
Now that you know how mothers fare [ie, go through pain]
Although you are a pure maiden human.”)
Since I mentioned the East, though, I will quote one of the Eastern Fathers of the Church:
“His birth was in accordance with the laws of parturition, while in that it was painless it was above the laws of generation. For as pleasure did not precede it, pain did not follow it; according to the prophet who says, “Before she was in labor, she brought forth,” and again, “before her pain came she was delivered of a man-child” (Isaiah 66:7).”
(St. John Damascene, On the Orthodox Faith, Book IV, chapter 14)