Basically , their answer is ‘magic’. The original 1988 version of Insight said under the entry for Jesus:
Since actual conception took place, it appears that Jehovah God caused an ovum, or egg cell, in Mary’s womb to become fertile, accomplishing this by the transferal of the life of his firstborn Son from the spirit realm to earth. (Ga 4:4) Only in this way could the child eventually born have retained identity as the same person who had resided in heaven as the Word, and only in this way could he have been an actual son of Mary” … “From the results revealed in the Bible, it would appear that the perfect male life-force (causing the conception) canceled out any imperfection existent in Mary’s ovum, thereby producing a genetic pattern (and embryonic development) that was perfect from its start.
The entire passage was deleted in the 2018 revision.