Luke 1:39-41
"39 In those days Mary set out and went with haste to a Judean town in the hill country, 40 where she entered the house of Zechariah and greeted Elizabeth. 41 When Elizabeth heard Mary’s greeting, the child leaped in her womb."
Please note the events above occurred within a week of the Conception of Christ, while John was six months in the womb of Elizabeth.
Making an unborn baby basically the first person to respond to the news of the Messiah’s coming. (Presumably after Mary of course.)