I HAVE learned one EXTREMELY important bit of information that I tell all my female running friends about - and it is the hormone Relaxin and it's role on the "weakening" of ligaments, during ones' cycle. Here is an article about it - skip down a couple paragraphs to see what I am talking about: http://www.wisegeek.com/what-is-relaxin.htm I had one friend run her first marathon while menstruating and she incurred hip injuries that it took her literally years to recover from - and I am convinced that is why. She is re-training now and I "reminded" her about this - and like lots of highly competitive females, she said, "I'm not going to pass on a race that I spent four months training for, just because my period started", to which I "gently" reminded her that the cost of forfeiting a race, is far less expensive than dealing with an injury that you will likely pay for, for the rest of your life." To which, she blew me off again, attempting ridiculous justifications such as, "you are more likely to incur damage from dehydration during a race than from your hormones" -this is assanine and she knows it! There IS a compromise to this dilemma - which I told her about. You sign up for TWO marathons one or two weeks apart from each other. People are always desperate to get into large competitive races -especially marathons- so you will most likely recoup the extra money you paid. And if you don't - you still only have one body...it's a small price to pay in the bigger picture. When I run my first ultra, some time before I die, I AM going to sign up for two. Problem solved.