In yoga, your menstrual, or "moon" cycle, is considered to be part of apana, or "outward flow" of energy, when the blood that once lined your uterus is being released so the cycle of fertility can begin again. Inversions are poses where your pelvis is higher than your heart, which can be excellent for your circulation. "However, when you are in your moon, inversions are going against the flow due to the effect of gravity on the blood vessels of the uterus," says Shiva Rea, a Yoga Journal contributing editor who teaches at Yoga Works in Santa Monica, California. "Your menstrual flow may lessen for a brief period before resuming its rhythm."
During your cycle, Rea suggests replacing inversions with restorative poses like Supta Baddha Konasana (Reclined Bound Angle Pose) and Salamba Upavistha Konasana (Supported Open Angle Pose), with blankets and bolsters under your belly to bring ease and balance to your entire being; these help you to relax and gently open the lower belly, pelvic floor, and womb. She also recommends the book Relax and Renew: Restful Yoga for Stressful Times (Rodmell Press, 1995) by Judith Lasater, Ph.D., which has a section on other restorative poses for the "moon club." Keep in mind that an important part of being in your moon flow is learning to let go—even of inversions—in order to replenish yourself.