Macroscopic examination of the placenta and histological examination of the membranous partition between amniotic sacs was peformed in approximately 1400 twin maternities in Ibadan, Nigeria.

Monochorionic placentation is mainly characterised by two layers (as opposed to four in the dichorionic one) in the dividing septum between the two amniotic sacs, as well as by the absence of a membranous ridge of tissues at the base of the septum, that is instead present in the dichorionic one.

Monochorial twins appear to be invariably MZ, whereas dichorial twins may be either MZ or DZ. In the present study the chance of being DZ was 95% in the case of single and of 99% in the case of double dichorionic placentation.