May girls exercise when they r in their periods ?

Certainly: It's fine to exercise during menstrual periods. Exercise often helps women feel better, improves energy, reduces pain & reduces pms. Athletic performance does not suffer. Some research suggests increased risk of knee injury during menses, but steps can be taken to reduce that risk. See www.Shape.Com/lifestyle/mind-and-body/exercise-and-your-menstrual-cycle-what-your-period-means-your-workout-schedu.