And the angel answered her, "The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be called holy—the Son of God."
Luke 1:35

Some children are of promise (Isaac, Joseph, et al), but only one child of promise can be called Son of God from conception. He is, was, and ever will be the Son of/within God. He was the Son before his body was conceived and he was the fulfillment of God's promises before the words were ever spoken.

And Mary said to the angel, “How will this be, since I am a virgin?”
Luke 1:34

Although Mary's question sounds like doubt, just as Zechariah doubted when he heard that he would have a son. The difference is that Mary asked "How will this happen?" while Zechariah asked "How can I know that you're telling me the truth?"

https://endtimeheadlines.org/2....022/11/father-of-mod