Luke 1:35 [35]And the angel answered and said unto her,…

Luke 1:35
[35]And the angel answered and said unto her, The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God.

How do Trinitarians explain.the son being co eternal when not until the birth of Jesus is he to be called the Son of God