Like above posters, I would pay more attention to the logistics of traveling than anything else. You could always make a lighthearted comment, "I hope you don't mind I brought a +1" or something... totally up to you. Just be yourself, enjoy being in the interview, and honestly I don't see how that could effect your acceptance because by the time D-school will have started, you will already have given birth (what would concern me the most as an 'adcom', I think, would be to offer someone acceptance and have them leave for a few months for maternity leave). The only advice I can give is try not to let lethargy get the best of you. Hang in there and do your best to be energetic. Best of luck and congratulations on your pregnancy. 🙂
Random side-note, wear a ring regardless because I think adcoms who may be older, may also be old-fashioned and if you're not wearing a ring (maybe it doesn't fit? maybe you left it at home?) could judge negatively. Small detail, not wanting to offend anyone, but just a side thought. Again, best of luck. 🙂