From Pharmacognosy (best book on this I've come across is Pharmacognosy by Trease & Evans), yams contain diosgenin, a steroidal sapogenin (triterpenoid). In the body, this is converted to sex hormones such as estrogen and progesterone.
For the same reason, communities with a high consuption of yams seem to have a higher incidence of multiple births. A particular community in West Africa (I forget which one, it's been a long time) is reported to have had over 75% incidence of multiple births principally for this reason.
Life is like playing a violin solo in public and learning the instrument as one goes on.