This is possible because high consumption of yam, which contain a natural phytoestrogen may stimulate the ovaries to release an egg from each side. No wonder the Yorubas present the highest dizygotic twinning rate in the world (4.4% of all maternities) at 45-50 twin sets (or 90-100 twins) per 1,000 live births. Dizygotic means fraternal, that is they developed from two separate eggs that are fertilized by two separate sperms.