Twins and More

Abstract This chapter addresses multiple births, with their biological quirks and marginal status in perinatal research. Multiples comprise a tiny percentage of all births and differ in many ways from singletons, with lower birthweights, shorter gestations, and higher rates of mortality. Because of these differences, multiples are often excluded from epidemiologic analyses, even as they remain a worthy topic of study in their own right. Twins have been a particular focus for the study of genetic contributions to health. While identical twins start with virtually identical genetic inheritance, they differentiate over time with the occurrence of somatic mutations and divergent epigenetic patterns. The occurrence of monozygotic twins is relatively constant in populations, apparently the product of chance events early after fertilization. The occurrence of dizygotic twinning is more susceptible to external factors, in particular, maternal age. Dizygotic twins are more common in people of African descent and less common in persons of Asian descent.