We examine whether mothers with greater health stocks and mothers who behave in a healthier manner during pregnancy are more likely to give live birth to twins. We have collected a number of microdata sets from various sources and countries, and in each case are collating variables measuring mother's health outcomes or behaviours, as well as whether the mother has ever given birth to twins. Using a number of statistical tests, we will examine whether those mothers who give birth to twins look different to mothers who don't give birth to twins, and if so, whether twins are more likely to occur when mothers have greater health endowments.