ABSTRACT
The importance of the pre-and postnatal determination of chorionicity, amnionicity and zygosity is outlined in numerous studies in the literature and described in detail in Chapter 24 of this book. According to the ‘Declaration of rights and statement of needs of twins and higher order multiples’, ‘every twin and their parents have the right to expect accurate recording of placentation and the diagnosis of zygosity of the same-sex multiple at birth’1. In the past, before the advent of ultrasound and DNA-based zygosity studies, the classification of so-called ‘fraternal’ or ‘identical’ twins was mainly a postpartum event.
