Hi Nathan,

Thanks for the very good question.

Mary was the Mother of the physical-earthly Jesus, not any non-physical eternal Jesus.

Now one could on and ask:

• Was Mary the Mother of Jesus before Jesus was born?
  

The Church would say No, she was not. Before Jesus was born, He was of one substance with the Father eternally begotten of Him.

Through the Incarnation, Mary conceived Jesus, the God-Man, through the power of the Holy Spirit.

Arianism taught the Son of God came to be from things that were not and that He was from another substance than that of the Father.

The Nestorianism heresy regarded Christ as a human person joined to the divine person of God's Son, while the Church teaches Jesus [was/is] the Divine Person.

Mary gave birth to the Divine Person, Jesus. To say Mary gave birth to a human person is a basic Christological error.

My colleagues may have more to add.

I hope this answers your question.

Mike