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.

When the level of sex chromatin in a newborn female remains as low as 2-8% after the first few days of life, some genetic disorder may be involved.

A clinical case is presented where only one of MZ twins appears to be affected.

A total of 70 karyotypes of infants from triplet pregnancies and their parents from 17 families were examined. The karyograms of 16 families showed a normal number of chromosomes and no abnormal morphology. Only in the 17th family a girl showed an additional chromosome in group G and deletion of the long arms in another from group C. A high concentration of developmental malformations was however present in this family.