In Star Trek Voyager “Friendship One” (season 7, episode 21), a heavily pregnant Torres and Paris are arguing about her going down to an irradiated planet. Paris does not want her to go because she is 6 months pregnant, the atmosphere was toxic and that she was “breathing for two”. She agrees not to go on the condition that the next time they are pregnant, that Paris carry it and she can go on away missions.
Is this something that is possible in the Star Trek universe?
Certainly, it's possible science has advanced far enough to allow men to carry children although I suspect the mechanics would be complex.
We know that the first recorded male pregnancy (in Star Trek) was that of Charles "Trip" Tucker in Star Trek: Enterprise in the episode Unexpected but even that wasn't a human/human pregnancy.
Tucker meets Ah'len, the Xyrillian engineer. They become ever more friendly while repairing the ship. Ah'len takes him to a holodeck to show him Thera, the Xyrillian homeworld. During the tour, Ah'len engages in a seemingly innocent game that allows them to share their thoughts and desires. When the repairs are completed and Tucker returns, he notices a strange growth on his wrist. He visits Phlox, who tells him that he is pregnant (though Phlox also states no genetic material is taken from the male involved).
T'Pol later reminds Tucker that his was "the first recorded instance of a human male pregnancy".
It seems unlikely, at least to me, that medicine would advance that quickly between Enterprise and Voyager but it's possible.
However, I suspect that B'Elanna's comment to Tom was intended more as a "You owe me one" comment rather than to be taken literally.
