The birth of Christ Jesus was unique among all births in that God's one and only Son became a man, and His mother was a young maiden girl who had never known a man (she was a virgin.) That it would be this way was revealed, by God, through the prophet Isaiah around the 7th century BC.
The verse below is what Isaiah wrote in the Old Testament of the Bible:
"The virgin will be with child and will give birth to a son, and will call Him Immanuel." 1
Immanuel is translated: "God with us."
The following is recorded in the New Testament of the Bible as to the birth of Christ Jesus:
"In the sixth month, God sent the angel Gabriel to Nazareth, a town in Galilee, to a virgin pledged to be married to a man named Joseph, a descendant of David. The virgin's name was Mary. The angel went to her and said, "Greetings, you who are highly favored! The Lord is with you."
Mary was greatly troubled at his words and wondered what kind of greeting this might be. But the angel said to her, "Do not be afraid, Mary, you have found favor with God. You will be with child and give birth to a son, and you are to give him the name Jesus. He will be great and will be called the Son of the Most High. The Lord God will give Him the throne of His father David, and He will reign over the house of Jacob forever; His kingdom will never end."
"How will this be," Mary asked the angel, "since I am a virgin?"
The angel answered, "The Holy Spirit will come upon you, and the power of the Most High will overshadow you. So the Holy One to be born will be called the Son of God." 2
1 (Isaiah 7:14)
2 (Luke 1:26-35)