There is a lot of stuff about this on the net here is what I found:
http://timesofindia.indiatimes.com/articleshow/1539606.cmshttp://multiples.about.com/od/funfacts/tp/howtohavetwins.htmhttp://multiples.about.com/cs/funfacts/a/oddsoftwins_2.htm
and a blog:
http://www.pregnancy-info.net/QA/answers-Natural_Ways_to_increase_chance_of_twins/
