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.
DH wore the same top to both our boys births and I was saying the other day that I should pit it away so it's not too worn out for the next one!

The maternity wards are quite stuffy so something light like cotton jammies that you can unbutton for easy BB access. I ended up asking DH to bring in some clothes for me as I got bored with pjs and wanted to wear trackies and normal clothes.