The Elizabeth, Zechariah, and the prophets weren’t the only ones who knew that Jesus would be born as their King. Mary and Joseph knew too.
In the sixth month the angel Gabriel was sent from God to a city of Galilee named Nazareth, to a virgin betrothed to a man whose name was Joseph, of the house of David. And the virgin's name was Mary. And he came to her and said, “Greetings, O favored one, the Lord is with you!” (Luke 1:26-28 ESV)
Mary found out even before she knew she was pregnant. Gabriel, the angel, was sent to her – letting her know what would happen.
But she was greatly troubled at the saying, and tried to discern what sort of greeting this might be. And the angel said to her, “Do not be afraid, Mary, for you have found favor with God. And behold, you will conceive in your womb and bear a son, and you shall call his name Jesus. He will be great and will be called the Son of the Most High. And the Lord God will give to him the throne of his father David, and he will reign over the house of Jacob forever, and of his kingdom there will be no end.” (Luke 1:29-33 ESV)
Yes, Mary knew that Jesus would be King. Joseph would find out in a similar way, but only after finding out his fiancée was pregnant.
And her husband Joseph, being a just man and unwilling to put her to shame, resolved to divorce her quietly. But as he considered these things, behold, an angel of the Lord appeared to him in a dream, saying, “Joseph, son of David, do not fear to take Mary as your wife, for that which is conceived in her is from the Holy Spirit. She will bear a son, and you shall call his name Jesus, for he will save his people from their sins.” (Matthew 1:19-21 ESV)
Joseph, too, knew that Jesus would be King. They both learned that he would be more than King, though. He would save his people from their sins. He would be called the Son of the Most High (God). He would be God with us.
All this took place to fulfill what the Lord had spoken by the prophet: “Behold, the virgin shall conceive and bear a son, and they shall call his name Immanuel” (which means, God with us). (Matthew 1:22-23 ESV)
How about you? Does the virgin birth help your faith in Jesus? Do you see him as your King? Do you see him as your Savior? Do you see him as God with us?