Every woman is different. Regarding myself (Anu Visuri, Gotta Joga's yoga teacher), I don’t notice anything even if doing a handstand during my period. I know women who say that inversions may cause a stop to the bleeding and then make the bleeding stronger and with cramps afterwards. If you have a painful or heavy bleeding during your  period, then it is better to avoid inversions, intensive backbends and abdominal muscle exercises and in general any intensive asana practice. Better to practice yin yoga or restorative yoga in that case (Note- Gotta Joga Evening). Downward facing dog however, may relieve menstrual pain so it can be done without concern. Twists and forward bends are also good asanas during the period.

In fact, the banning of yoga or certain asanas during the period is originally a male invention, and certainly cannot be generalised to all women.We women need to learn to listen to our body and learn what is good for us and what not. Every yoga style or school have different rules: e.g. in ashtanga yoga women are forbidden to do an asana practice during the first 3 days of her period, Iyengar forbid the inversions and backbends during the period and abdominal exercise as a whole from women. No man can know the female body as well as a women herself. And that is exactly why to practice yoga:  to get to know our body and mind.