Even if John the Baptist were cleansed of original sin when he lept in Elizabeth’s womb, he still would have first contracted original sin when he was conceived/ensouled. So he wouldn’t have been an exception to the universality of original sin. I’m not sure exactly how they do this but I believe baptism can be administered to a baby still in the womb – it’s mentioned in Catholic texts that this can be done if the baby is in danger of dying.
BTW, there is a theological strain in Catholic tradition that John the Baptist was sinless (not at all a dogma of course) (see below for documentation of this) I don’t think it is stated though that he is free of concupiscience – whereas the Blessed Virgin Mary was so and was furthermore according to the traditional common teaching incapable of sin (impeccable see:
“Theologians assert that Mary was impeccable, not by the essential perfection of her nature, but by a special Divine privilege. Moreover, the Fathers, at least since the fifth century, almost unanimously maintain that the Blessed Virgin never experienced the motions of concupiscence.”
newadvent.org/cathen/15464b.htm
To assure you that it is fine to believe that John the Baptist was cleansed of original sin before being born (as I understand it some babies have been by baptism) let me quote also from the Catholic Encyclopedia:
““And it came to pass, that when Elizabeth heard the salutation of Mary, the infant” – filled, like the mother, with the Holy Ghost – “leaped for joy in her womb”, as if to acknowledge the presence of his Lord. Then was accomplished the prophetic utterance of the angel that the child should “be filled with the Holy Ghost even from his mother’s womb”. Now as the presence of any sin whatever is incompatible with the indwelling of the Holy Ghost in the soul, it follows that at this moment John was cleansed from the stain of original sin.”
newadvent.org/cathen/08486b.htm
Here’s St Catherine of Sienna on John the Baptist’s sinlessness:
“John the Baptist never sinned and was sanctified in his mother’s womb”
ccel.org/ccel/catherine/dialog.iv.iv.ii.html
IIRC there’s also a strain in Catholic tradition that says St Joseph never sinned but I couldn’t find anything on the web about it except this anecdotal story:
homeschoolalumni.org/viewtopic.php?t=4395&start=30
(this is also not a dogma)