Yams, a type of sweet potato, have been linked with multiple births. A tribe
in Africa whose diet consists mostly of
yams was recently found to have exceptionally high rates of twins and multiple births. Yams are thought to contain chemicals that stimulate hyperovulation, increasing your chances of conceiving twins.
Fern Reiss' Infertility Diet also recommends eating yams, that they are sort of nature's Clomid and help trigger the release of the egg (or eggs).