This looks to be an incredibly helpful article:

http://www.healthyhormones.com/news/news02.htm

"It does not matter whether or not diosgenin (the active sapogenin in Mexican wild yam) converts to any sex hormones in the body. There is still cellular activity from the almost identical steroid structure of the diosgenin molecule. Clearly, there is a relationship between the structure of the phytohormones (diosgenin from Mexican wild yam, and genistein from soy) and the naturally occurring sex hormone estradiol (E2). This more than likely explains why wild yam creams can have such a positive effect on some women even when used without Progesterone."