KJV - 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.
NLT - The angel replied, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you. So the baby to be born will be holy, and he will be called the Son of God.
A personal note:
Jesus was not called 'a' Son of God, but 'the' Son of God. For me, it is a distinguishable difference. We are all children of God, adopted into His family upon salvation. Jesus, however, is THE Son of God, Who was born into this world to save us in spite of ourselves.