It seems that the Council of Trullo (aka the Quinsext Council), one of the canons (79) dealt with the incorrect pious celebration of the “childbed” (or puerperia) of Mary:
As we confess the divine birth of the Virgin to be without any childbed, since it came to pass without seed, and as we preach this to the entire flock, so we subject to correction those who through ignorance do anything which is inconsistent therewith. Wherefore since some on the day after the holy Nativity of Christ our God are seen cooking σεμίδαλῖν , and distributing it to each other, on pretext of doing honour to the puerperia of the spotless Virgin Maternity, we decree that henceforth nothing of the kind be done by the faithful. … If therefore anyone henceforth be discovered doing any such thing, if he be a cleric let him be deposed, but if a layman let him be cut off.
“Childbed” is the “accidents” (if you will) of childbirth: pain, blood, afterbirth, etc. (as the commentary of Zonaras explains). Now, what is the status of this “confess[ion]” of the Council of Trullo? Is it doctrinal? It occurred in AD 692, several centuries after St. Jerome wrote the following in Contra Helvidium, n. 21:“Are virgins better,” you ask, “than Abraham, Isaac, and Jacob, who were married men? Are not infants daily fashioned by the hands of God in the wombs of their mothers? And if so, are we bound to blush at the thought of Mary having a husband after she was delivered? If they find any disgrace in this, they ought not consistently even to believe that God was born of the Virgin by natural delivery. For according to them there is more dishonour in a virgin giving birth to God by the organs of generation, than in a virgin being joined to her own husband after she has been delivered.” Add, if you like, Helvidius, the other humiliations of nature, the womb for nine months growing larger, the sickness, the delivery, the blood, the swaddling-clothes. Picture to yourself the infant in the enveloping membranes. Introduce into your picture the hard manger, the wailing of the infant, the circumcision on the eighth day, the time of purification, so that he may be proved to be unclean. We do not blush, we are not put to silence. The greater the humiliations He endured for me, the more I owe Him. And when you have given every detail, you will be able to produce nothing more shameful than the cross, which we confess, in which we believe, and by which we triumph over our enemies.
Now, certainly St. Jerome was not writing infallibly, and he does not necessarily admit that Mary’s pregnancy was accompanied by “the sickness” of pregnancy and “the blood” of childbirth (= childbed), but merely offers those as “humiliations” Jesus could have endured in his assuming our nature.
Add to this the testimony (?) of Luke 2:22-24, depending on your translation. The sacrifice of two turtledoves for “her” (Mary’s) or “their” (Mary’s and Jesus’s) “purification” according to the Law of Moses. Since we believe Mary to be free from the stain of sin, and Jesus was certainly as well, apart from showing their submission to the Law, what was the purpose of this purification?
So my question is: has Mary’s painless childbirth actually been defined, or is it just the sensus fidelium? Trullo anathematizes those who “celebrate” Mary’s childbed, but it does not seem to impose the belief.