By His Fiat, she gave birth to Him, at His request.
Why to declare 1John4 which is the Test of the true Christ.
1 John 4
New American Standard Bible (NASB) Testing the Spirits
4 "Beloved, do not believe every spirit, but test the spirits to see whether they are from God, because many false prophets have gone out into the world. 2 By this you know the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is from God; 3 and every spirit that does not confess Jesus is not from God; this is the spirit of the antichrist, of which you have heard that it is coming, and now it is already in the world. 4 You are from God, little children, and have overcome them; because greater is He who is in you than he who is in the world. 5 They are from the world; therefore they speak as from the world, and the world listens to them. 6 We are from God; he who knows God listens to us; he who is not from God does not listen to us. By this we know the spirit of truth and the spirit of error."
Whether we like it or not God wanted a mother by Birth of a woman named Mary. He wanted a "vessel" to be human/flesh. Yes he even a became a Baby. Why to declare 1John4 which is the Test of the true Christ. Divinity became man by a woman. He was "clothed" into humanity by this action. Since he came into humanity as divinity.Which is why he is God thus mother of God. Mary as mother is humanity/flesh to the divinity still intact. It does not mean she was before God. In a nutshell she brought forth the human/flesh and Divinty of Jesus as human baby. She is the mother of God from his human birth. His divinity is intact as he is human/flesh as the scripture reads in 1 John 4.