θεοτόκος Defnintion

(Emperor Justitian) has pointed out, as you have learned from the contents of his letter, that disputes have arisen over the following three questions: Can one say that Christ our God is "one of the Trinity", that is one holy person among the three persons of the Holy Trinity? Did Christ our God who in His divinity is impassible suffer in the flesh? Must Mary, the ever Virgin Mother of our Lord and God Jesus Christ, be called truly and properly Mother of God and Mother of God the Word incarnate from her?...
Christ is one of the Holy Trinity, that is one holy person or subsistence (subsistentia)--one hypostasis as the Greeks say--one among the three persons of the Holy Trinity.

(There follow among other quotations: Gen. 3.22; 1 Cor. 8.6; the Nicene Symbol).

The fact that God did truly suffer in the flesh we confirm likewise by the following witnesses (Deut. 28.66; Jn 14.6; Mal. 3.8; Acts 3.15; 20.28; 1 Cor. 2.8; Cyril of Alexandria, Anathematism 12; Leo I, Tome to Falvian, etc.).

Constantinople 4th-13th Centuries

We teach that it is right for Catholics to confess that the glorious and holy Mary, ever Virgin, is truly and properly the Mother of God and Mother of God the Word incarnate from her. For it is He himself who truly and properly became incarnate in these latter days and deigned to be born of the holy and glorious Virgin mother. Hence, since the Son of God became incarnate and was born from her truly and properly, we confess her to be truly and properly the Mother of God incarnate and born from her. In proper terms, lest one should believe that the Lord Jesus Christ received the name of God as a title of honor or as a favor as Nestorius foolishly taught; truly, lest one should believe that from the Virgin he took on a mere appearance of flesh or in some other way a flesh which was not real, as Eutyches irreverently declared.

Pope John II, Letter to the Senate of Constantinople (534)
Dupuis, The Christian Faith, Alba House: New York, 1990, #617 (p. 167).

Jesu Redemptor Omnium (Christmas, Hymn)

Ave Maris Stella

There are a couple of other traditional Breviary hymns of this Feast of Mary's Maternity 

Cælo Redémptor prætulit

Te, Mater alma Núminis