Catechism of the Catholic Church
b. Para. 495: Called in the Gospels "the mother of Jesus," Mary is acclaimed by Elizabeth, at the prompting of the Spirit and even before the birth of her son, as "the mother of my Lord" (Lk 1:43; Jn 2:1; 19:25; cf. Mt 13:55, et al) In fact, the One whom she conceived as man by the Holy Spirit, who truly became her Son according to the flesh, was none other than the Father's eternal Son, the second person of the Holy Trinity. Hence the Church confesses that Mary is truly "Mother of God" (Theotokos) [Council of Ephesus (431): DS 251].
Para. 468: After the Council of Chalcedon, some made of Christ's human nature a kind of personal subject. Against them, the fifth ecumenical council of Constantinople in 553 confessed that "there is but one hypostasis [or person], which is our Lord Jesus Christ, one of the Trinity" (Council of Constantinople II [553]: DS 424).
Para. 503: ". . . He is naturally Son of the Father as to his divinity and naturally son of his mother as to his humanity, but properly Son of the Father in both natures" (Council of Friuli [796]: DS 619; cf. Lk 2:48-49).