John Lightfoot on Luke 1:41

[The babe leaped in her womb.] So the Seventy, Genesis 25:22, the children leaped in her womb. Psalm 114:4, the mountains skipped. That which is added by Elizabeth, verse 44, the babe leaped in the womb for joy, signifies the manner of the thing, not the cause: q.d. it leaped with vehement exultation. For John, while he was an embryo in the womb, knew no more what was then done, than Jacob and Esau when they were in Rebekah's womb knew what was determined concerning them.

"At the Red Sea, even the infants sang in the wombs of their mothers"; as it is said, from the fountain of Israel Psalm 88:27; where the Targum, to the same sense, "Exalt the Lord ye infants in the bowels of your mothers, of the seed of Israel." Let them enjoy their hyperboles.

Questionless, Elizabeth had learned from her husband that the child she went with was designed as the forerunner of the Messiah, but she did not yet know of what sort of woman the Messiah must be born till this leaping of the infant in her womb became some token to her.