I started out wearing a light weight loose, longish (down to mid thighs) night t-shirt with a scoop neck. Although for both of my births experiences I ended up stripping it off at some point (I have no recollection of actually doing that, just the pics of me with it on and then off).
I found that as I progressed with labor I did not want to feel any clothing constricting me, even the very light weight t-shirt I had on. I just needed to feel free in my body. Of course, I did have 2 home births, so I was very comfortable with nothing on.
i had my daughter at a birth center and i just wore a cheap tank top that i had and didnt care if it got ruined plus it was black so really couldn't see anything if it was ruined and since i had a water birth there really wasnt a mess and when my daughter was out my midwife took the top off of me so that we could be skin on skin i saved the tank top for the next birth i just put it with all of my other maternity clothes
I had a home birth and just wore a sports bra most of the time. Although, near the beginning of my labor when I wasn't in the tub, I wore a tank top and some sweat pants. Just wear whatever you are most comfortable in.
I almost went shopping for something to wear during labor, but decided not to, and I'm glad I didn't. It wouldn't have been worth the money.
I just bought a bathing suit top and a bathing suit skirt which I need to cut the panties out of. This way I can walk around the house before getting in the tub and not be shy about being video taped and photographed. Total spent at Target about $40. This is the birth top/bathing suit I just got.