Ok great I'm the weird one here LOL!! Since I knew I wanted more than one child, when I was pregnant with Matthew I bought one of those "football style" nighties with a number on the front - a huge 90 for the year he was born. I have worn the same nightie for each of my births and it's packed on top in my labour bag for this time as well.
As for in the hospital, I alternated between nighties and trackies depending on how I was feeling. Just having the option of both available made me a little more comfortable since I'm a horrible patient anyway. I always take the undies I don't care about that are nice and roomy so if they get too messy I can just toss them.