44 For behold as soon as the voice of thy salutation sounded in my ears, the infant in my womb leaped for joy.
Augustine: In order to say this, as the Evangelist has premised, she was filled with the Holy Spirit, by whose revelation undoubtedly she knew what that leaping of the child meant; namely, that the mother of Him had come to her, whose forerunner and herald that child was to be.
Origen: From that time Christ ordained John to be a prophet.
45 And blessed art thou that hast believed, because those things shall be accomplished that were spoken to thee by the Lord.
Theophylact: The mother of our Lord had come to see Elisabeth, as also the miraculous conception, from which the Angel had told her should result the belief of a far greater conception, to happen to herself; and to this belief the words of Elisabeth refer.
Amborse: Mary doubted not but believed, and therefore the fruit of faith followed.
Bede: Nor is it to be wondered at, that our Lord, about to redeem the world, commenced His mighty works with His mother, that she, through whom the salvation of all men was prepared, should herself be the first to reap the fruit of salvation from her pledge.
Ambrose: Happy are you also who have heard and believed, for whatever soul has believed, both conceives and brings forth the word of God, and knows His works.
Gregory: She was touched with the spirit of prophecy at once, both as to the past, present, and future. She knew that Mary had believed the promises of the Angel; she perceived when she gave her the name of mother, that Mary was carrying in her womb the Redeemer of mankind; and when she foretold that all things would be accomplished, she saw also what was as to follow in the future.
Scripture from the Douay-Rheims Bible. Commentary from St. Thomas Aquinas, Catena Aurea, Vol. III (London: Rivington, 1843).