"The babe leapt in her womb"
Altarpiece depiction of the Visitation (c.1434-35)
Public DomainAltarpiece depiction of the Visitation (c.1434-35) - Credit: Jacques Daret

This is a slightly modified quotation from the Authorized King James Version of the Bible's New Testament (Luke 1:41):

And it came to pass that when Elisabeth heard the salutation of Mary, the babe leaped in the womb; and Elisabeth was filled with the Holy Ghost.

It is a reference to the visit by the pregnant Mary, mother of Jesus, to Elisabeth, mother of John the Baptist, whilst she too was pregnant. The event is generally known as the Visitation.