The incidence of twinning in Western Nigeria is very high, being 45‰ (Nylander, 1969a), as compared with 11-12‰ in Caucasian populations (Registrar General, 1967). A recent study of twinning in Ibadan, in which zygosity of individual twin pairs was determined, has shown that the pattern of placentation and zygosity in this population is also very different from that in Caucasian populations.
As many as 1475 consecutive newborn twin pairs delivered in three hospitals in Ibadan between March 1967 and April 1969 were investigated.
The placentae of the twins were examined macroscopically by the author personally to determine the form (whether single, fused, double or separate) and the membrane relationships (whether monochorionic or dichorionic) as shown in Figs. 1 and 2. The membrane relationship was confirmed by histological examination.
Cord blood samples were collected from each twin pair and studied for (1) ABO, Rh (using antisera, C, c, D, E, e), MNSs and Gonzales blood groups and (2) G6PD electrophoretic pattern. Other blood groups like Fya, k and Jk, used initially, were discontinued later, because the gene frequencies in this population are such that they are uninformative for zygosity determination.
Samples of the placenta of each twin were sent in refrigerated containers to the Galton Laboratory, London University, for placental enzyme studies — phosphoglucomutase and peptidase (Hopkinson and Harris, 1968; Lewis and Harris, 1967).