I agree with the other reviews for this episode. It is very, very bad indeed. Nothing seems to tie up with accepted ST continuity or even everyday common sense. How could Janeway evolve, mate, gestate and then give birth within the space of just three days? And Chakotay's decision to simply abandon fledgling human genetic material on a pre-warp planet must rank as the most grotesque violation of the prime directive ever - and this is right after Janeway lectured him about the absolute importance of that same directive in the previous episode ('Alliances' season 2 episode 14).
Season 2 of Voyager features absolute classic episodes such as 'The 37's', 'Projections' and my favourite Voyager episode of all 'Non Sequitur' but there were also moments in the first two seasons of Voyager that are disturbingly reminiscent of 'Voyage to the Bottom of the Sea' at its worst. 'Threshold' is like a an episode of VttBotSea but with a bit of reproduction added in for good measure. Normally it was Kowalski who turned into a monster, only to shake his head at the end of the episode - quite miraculously back to his old self (with no lingering side effects, of course). Here, Paris and Janeway are channeling the spirit of Kowalski!