I refuse to buy new bike shorts since they cost a fortune in the first place, so I've been figuring out which pairs feel the best. My most favorite pair are my Coeur Trishorts (http://www.coeursports.com/collections/triathlon/products/sea-glass-triathlon-shorts). Not only is the chamois super comfortable, but the waist band doesn't put any added pressure and seems to keep feeling great as the baby bump gets more pronounced. Also, the band isn't too low that I don't feel like I'm sticking out and covers the belly nicely (not maternity style, but just right) Thanks to a teammate (Amy Farrell) who is on the Coeur Team I was able to get the shorts at a very reasonable price when I was in Kona. I never wore them before being pregnant, but I can assume they feel just as great during Ironman weight. Kebby (the owner) seems to really understand women and always has a smile!
My other favorite shorts are the Zoot Performance Trishort (http://zootsports.com/womens/apparel/w-performance-tri-4-short-s14). The waistband and legs feel fantastic. Sometimes the chamois seems hurt a little with my growing hips, but overall very comfortable. The waist band seems to roll down slightly as my bump gets bigger, not sure how much I like that. I wore these through Ironman training, and they were one of my favorites, not a surprise I still like them a lot.
The worst shorts from the very beginning have been the Betty Designs Trishorts (http://www.bettydesigns.com/collections/triathlon-1/products/garden-party-womens-tri-short)...these aren't the exact shorts I have, mine are the HPT custom design, but similar style. The waistband digs in and doesn't give myself or the baby any room to breathe! I didn't like them much at Ironman weight, always lots of chaffing. I was shocked at how uncomfortable they were in the first 6 weeks!
I have an older pair of Pearl Izumi shorts that were fine for the first 4.5 months, but the waistband is too low and just doesn't feel right anymore!
I write this as I'm getting an easy ride on the trainer :)