Catechism of the Catholic Church §499 — English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va (official text, quoted at the cited paragraph)
The deepening of faith in the virginal motherhood led the Church to confess Mary's real and perpetual virginity even in the act of giving birth to the Son of God made man. In fact, Christ's birth "did not diminish his mother's virginal integrity but sanctified it." And so the liturgy of the Church celebrates Mary as Aeiparthenos, the "Ever-virgin".
This text is under copyright; the cited paragraph is quoted for study, with a link to the full document. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Why Pitre cites it
Mary's virginity in giving birth — the doctrine behind the Fathers' painless birth, in the Catechism's words.
Notes
The Latin phrase "virginitas in partu" does not appear in the English text; the concept is expressed as "real and perpetual virginity even in the act of giving birth" and "did not diminish his mother's virginal integrity but sanctified it" (quoting Lumen Gentium 57). Vatican.va page has "and so" in lower case after the footnote marker; capitalised here to "And so" as in the printed edition. Footnote numbers 154–155 removed.