A question for you: Why do you suppose Mary "did not understand the saying which he spoke to them"?
Luke 2:51 And he went down with them and came to Nazareth, and was obedient to them; and his mother kept all these things in her heart.
Luke 2:8 And in that region there were shepherds out in the field, keeping watch over their flock by night. 9 And an angel of the Lord appeared to them, and the glory of the Lord shone around them, and they were filled with fear. 10 And the angel said to them, "Be not afraid; for behold, I bring you good news of a great joy which will come to all the people; 11 for to you is born this day in the city of David a Savior, who is Christ the Lord. 12 And this will be a sign for you: you will find a babe wrapped in swaddling cloths and lying in a manger." 13 And suddenly there was with the angel a multitude of the heavenly host praising God and saying, 14 "Glory to God in the highest, and on earth peace among men with whom he is pleased!" 15 When the angels went away from them into heaven, the shepherds said to one another, "Let us go over to Bethlehem and see this thing that has happened, which the Lord has made known to us." 16 And they went with haste, and found Mary and Joseph, and the babe lying in a manger. 17 And when they saw it they made known the saying which had been told them concerning this child; 18 and all who heard it wondered at what the shepherds told them. 19 But Mary kept all these things, pondering them in her heart. 20 And the shepherds returned, glorifying and praising God for all they had heard and seen, as it had been told them. RSV