Both the miraculous and uniqueness of Jesus' birth is found in the fact that His father is God and His mother was a young maiden girl who had never known a man (a virgin) - her name is Mary and as to what took place, the following is written in the New Testament of the Bible:

"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."1  That this would happen was also spoken of by the Prophet Isaiah over 600 years earlier; he wrote that a virgin would be with child and give birth to a son and that the child would be called Immanuel (Immanuel means God with us.)   This prophecy is recorded in the Old Testament of the Bible, the book of Isaiah, chapter 7, verse 14.

Bible Reference:

1 (Luke 1:26-35)

Grace to you and peace from God our Father and the Lord Jesus Christ.