Do you really burn any calories with prenatal yoga?

I can't do very much exercise right now being almost in my third trimester of pregnancy and all the snow we've been getting where I am, so all I have is my 50 minutes of prenatal yoga and the occasional game on Wii. I want to remain fit during my pregnancy, and I am sure my yoga work out doesn't burn that much at all, but I'd still like to know how many! Google isn't letting me in on it and I'm starting to feel guilty for trying to figure it out!