Even though Mary had not requested a sign, God gave her one, namely, the pregnancy of Elizabeth. The exact relationship between Mary and Elizabeth is unknown, but they obviously knew they were relatives. [source][source][source]
Gabriel also reassured Mary with one of the greatest statements of God"s power that God recorded in the Bible ( Luke 1:37). This verse undoubtedly comforted Mary in the following months as it has comforted countless believers faced with difficult ministries ever since. God can do the impossible (cf. Jeremiah 32:17; Jeremiah 32:27). Gabriel was alluding to the angel"s words to Sarah when he announced that she would bear a son in spite of apparently impossible obstacles (cf. Genesis 18:14). Luke 1:37 should also encourage readers of this story who doubt the possibility of a virgin birth to believe that God can do even this. [source][source][source]