The argument is that John's spirit was already in Elisabeth's womb and so when he 'recognized' the Savior, for whom he would prepare the way, he wiggled. Elizabeth gave birth three months later (v. 56), so the spirit must enter the womb by the beginning of the third trimester.Luke 1:41, 44 wrote:And it came to pass, that, when Elisabeth heard the salutation of Mary, the babe leaped in her womb; and Elisabeth was filled with the Holy Ghost:
...
For, lo, as soon as the voice of thy salutation sounded in mine ears, the babe leaped in my womb for joy.
The argument here is that since Jesus was talking to Nephi, he hadn't entered the womb, yet. So the spirit enters the womb just before, or at birth.3 Ne. 1:13 wrote:Lift up your head and be of good cheer; for behold, the time is at hand, and on this night shall the sign be given, and on the morrow come I into the world, to show unto the world that I will fulfil all that which I have caused to be spoken by the mouth of my holy prophets.
Further arguments can be made either way to negate the other, but really it's inconclusive.