“Virginitas in partu” is also tangentially related to the teaching that Mary is the sinless “New Eve” who chose to obey the Lord instead of rebelling and sneaking; and that therefore Mary did not suffer pain in childbirth (among other consequences). Since there are some women even now who suffer barely any pain (the luckies), it speaks to how God originally designed our bodies versus how they work after the Fall.
Here’s a small part of St. Ambrose’s big discussion of Mary’s virgin lifestyle, and his comparison to her as virgin mother as a sort of New Sarah:
""Come, Eve - now Sarah, since you bear children not in sorrow but in joy, not in grief but in laughter…
"…“Listen to Sarah, your wife.” (Gen. 21:12) … [Sarah,] your husband is bidden to listen to you.
"Now if by giving birth to a type of Christ, Sarah merits to be listened to by her husband, how great an advantage accrues to the [female] sex through its bringing forth Christ! And that, without loss of virginity!
"Come, then, Eve - now Mary - who has not only given us an incentive to virginity, but has also brought us God.
"… Illustrious, then, is Mary who bore aloft the sign of holy virginity, and raised on high for Christ the pious standard of inviolate chastity intemeratae integritatis].
“And yet, while all are invited to the practice of virginity by the example of holy Mary, there have been those who would deny that she persevered a virgin. We wished to keep silent about such a great sacrilege; but because in our midst, one called bishop [Bonosus of Nis] must also be argued with about this error, which we do not suppose will remain uncondemned…”
St. Ambrose, “De Institutione virginis et Sanctae Mariae virginitate perpetua,” lib. 1, c. 5, 32-33; 35.
Ambrose also talks about perpetual virginity in Epistle 42-43.
One of the main problems with studying this subject is that people aren’t comfy talking about it in English, and a lot of the great Victorian translators didn’t tackle these works because they couldn’t have been printed without using some very exact terminology.
But the Fathers talked about it, mostly in response to Bonosus’ new idea that Mary wasn’t a virgin all her life, or Helvidius’ idea that Mary had other kids.
“Intemerata” is one of the words talking partly about physical wholeness of virginity in this context, as is “integritas.” So if you’ve been singing all these years about “Mater amata, intemerata,” now you know.