I've written about this before, in two parts, in Elephant Journal. But I think it's time to write about it again in a concise and bullet-pointed way, because I see so many students who are pushing through and unaware of what they are doing to their bodies when they go full-force during this tender time of the month.
1. Your compromised for energy and already pushing yourself at a pace for the opposite sex because of society. The moon cycle is a time to rest and allow energy to stay inside the body and work its magic. We loose a lot of blood at this time and it is also a time of hormones shifting and constant muscle activity by the uterus. When we expect ourselves to engage at a "normal rate," we are denying the cyclical nature of our very lives. As women, we are endowed (not cursed) with these cycles that allow us to bring life into the world. And even if you don't want to do that, these cycles allow us, each month, to clear out a lot of old debris and energy from the body. So respecting just a few days as the cycle begins is the least we can do for ourselves. One way to do this is to ease up on practice (or focus only on cooling pranayama without retentions and seated meditation as well as chanting to settle the vata dosha) and cut out inversions. You are a woman, so embracing this will make it a LOT more fun!
2. Your pelvic region is not only more tender but is more "loose" during this time of the month, the energy is toward apana and you don't want to pull it up with the bandhas. The whole idea during the moon cycle is to flush out not only the uterus but also toxins in the body. It may be the point that the uterus is flushing out, but the apanic energy (falling under the umbrella of pranic energies) so too encourages a general flushing of the bowels and entire system. To practice at this time is to do so in the midst of a level of instability and softness in the pelvic area that I have experienced to encourage dislocation of the SI joint and general hyper-mobility leading to other injuries. Furthermore, when you lift more extremely with the bandhas (perhaps to jump back, jump through, etc.), you are pulling the pelvic floor up against the motion downward. This is all not to mention how putting your pelvis above your head for exorbitant periods of time causes your uterus to almost audibly groan at the distraction from all of her hard work.
3. This is your chance to heal and recuperate. Like point one above suggests, we are pushing ourselves full speed all month long, like the opposite sex. But our bodies, all month long, are fine-tuning so many more hormonal changes and physiological changes. Look around. A lot of the yoga instructors we turn to are men. Yet if you take an advanced female practitioner aside, chances are good she will tell you she doesn't practice full-on, if at all, on the first days of her cycle. And she's better off for it. I can say I was resistant to this idea at first as well. My ego was so hell-bent on practicing six days a week. But once I took those few, precious days to let my body release, process and heal, I found that once I hit the mat again, I was way stronger than ever, and my practice steadily improved at just the same rate as before, with less injuries and more of a sense of forgiveness around my gender!!!