I haven't done a tri while pregnant, or ever, but I have had 3 children, ran a couple marathons (not while pregnant tho), and I stay very fit while pregnant, so I will chime in.
Regarding nutrition, if it were me I'd focus more on protein than carbs. Try to hit at least 90 grams each day, tons of eggs, lots of dark veggies. And of course stay very hydrated and take in more salt than you would while not pregnant.
For the cycling shorts, I doubt you'd need maternity if you could find a pair that have a yoga-style waist and stay low on the belly. Or what about a full-body something or other?
Personally I'm of the belief that a woman can do just about anything in pregnancy she sets her mind to, with caution and moderation, and I think the balance thing is hogwash. Esp. at 23 weeks. For sure a man made that up. I never felt like I lost my balance in pregnancy. A sprint tri seems totally reasonable to me as long as you train well and listen to your body.
I once had a spin teacher who taught class her entire pregnancy and she kicked my butt even at 8 months pregnant. I have seen dancers who are the epitome of grace - with huge bellies sticking out their front.
In the second trimester, women usually feel *great*. It is the BEST time of pregnancy, and if you want to do a tri, it might be your only chance for quite some time.
~ working mom to 3 little girls ~
Roadie... 2010 54cm Trek Madone 4.5, Bontrager inForm