Mary saw the angel. 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 bring forth a Son, and shall call his name Jesus. He will be great, and will be called the Son of the Highest; and the Lord will give 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.
Usually when we hear good news, we are surprised, not fearful. However, the weight of this conversation was unexpected and unheard of at this time.
Mary knew that she was officially betrothed/engaged to Joseph only. She was concerned about the appearance as this situation unfolded. The angel outlined that the Holy Spirit would come upon her and the power of the Highest would overshadow her. The Holy One born would be the Son of God.
How many times do we think about something that's impossible or far reaching? Fear creeps in when we are not sure God is aware and directing the outcome. Ask God for grace to stay in faith until manifestation occurs.