Here is a study that suggests that by practicing yoga during the two weeks leading up to menses that cramps can be reduced.
Cobra, Cat and Fish Yoga Poses for Dysmennorrhea
Ninety-two young women (aged 18-22 years) with primary dysmenorrhea were randomly assigned to a yoga or control group. The yoga group was asked to practice Cobra, Cat and Fish poses during the Luteal phase of their cycles. No intervention was practiced during the first menstrual cycle. The yoga group practiced yoga for the next two cycles and the control group did not receive any intervention. The Visual Analog Scale for pain was used to access intensity and duration of pain. In the yoga group, there was a significant difference in pain intensity and duration post-intervention compared to pre-intervention and to the control group.
J Pediatr Adolesc Gynecol. 2011 Aug;24(4):192-6. Epub 2011 Apr 21.
Effect of three yoga poses (cobra, cat and fish poses) in women with primary dysmenorrhea: a randomized clinical trial.