Thank God for elastic! I was able to perform up until 35 weeks. I had some costuming that seemed to be a bit more giving. I had a coin bra that was too big (I had to pad it pre-pregnancy). I've just been able to make due. My Ghawazee coats no longer fit, but before/after performances, I would just wrap a dupata around me.

Good luck!

i performed at 6.5 months in a banded LRose dance bra...i have just ordered 2 more in a larger size because they are soooo comefy...i made a trumpet skirt to match because i think they are really flattering.

I just found a Melodia "twist top" that is designed to support breasts like a sports bra would. It has a flattering cut & is one size fits all (up to a 38D i think it says). I have not recieved it in the mail yet to attest if it will adjust well to my growing boobs & even better fit after breastfeeding when everything goes back to semi regular size...but i hope so.
I can't afford to buy tops that will only fit for a short period of time.

Oh, I love the twist tops and dance bras like L. Rose has...way comfy. When I was nursing last time (couple of years ago) and performing at Ren Faire our troupe used a black halter style swim top (not shiny, lol!), embellishing the bottom band and along the cleavage line. It looked so nice and was so much more comfy than underwire bras that we used them for all kinds of gigs!

The other thing I found really comfy and very useful (I don't know if you dance tribal, cab, or both) was a simple chiffon chemise-thingy with a cutout front (like a turkish vest that fits around the bra) - bit like a ghawazee coat I suppose, but with no seam down the front and very roomy rather than close-fitting. You can make them long with slits on the sides to allow for leg movement or shorter so it doesn't show much under the belt. Either way, it was a nice way to cover the belly but still shows torso movements really well. The only downside is that with my version you pretty much have to wear a vest with your bra to make it look right. I'm sure someone could come up with a more streamlined design on top which would negate the need for a vest.
If you look in my photo album I believe there is a picture of me doing a layback where I'm wearing it, so you can sort of get the idea of what I mean. I don't know if I have any better pictures. You could use black, white, or a color - the most important thing is to make sure the fabric you use is breathable, or else you're gonna get overly hot. A very light chiffon or thin cotton gauze works very well.

Somewhere out there I'm sure someone else is wearing the same thing or something similar, as I know I've seen pictures of another troupe in them at some point a long time ago.

Hope that helps!