Yoga: can you do inversions when you're menstruating?

One of the first rules I learned when I took up yoga was that I had to avoid inversions like headstand, shoulder stand and plow when I'm on my cycle. I never bothered to ask why. But after doing some research, I found out that it's thought that because inversions cause the blood to flow in the opposite direction, it can lead to disruptions in regular flow and even cause endometriosis. ouch!

But don't take this as gospel -- there are no studies to back this claim up. At the same time, there are no studies that show that inversions don't cause problems. So what to do? Listen to your body, and perhaps err on the side of caution.

What do you think?