The article by Fisk and Bryan (November 1993) highlights the confusion in the minds of obstetricians and paediatricians regarding chorionicity and zygosity. It is clear that pathological examination of placentae can be a confirmatory aide to the ultrasound determination of chorionicity and would enhance our understanding of the relationship between zygosity and chorionicity.

In 1968 Cameron examined the placentae from 668 twin pregnancies, accounting for over 90% of such pregnancies, over a two-year period in Birmingham. From an analysis of placental morphology, the sex of the infant and genotyping, he was able to demonstrate that 28% of the twin pairs were monozygotic. With regard to placental morphology, he found that 80% were dichorionic and 20% monochorionic. The percentage of monozygous twins was lower than the reported rates in caucasian population (39%) (Potter 1963) and also lower than the rate stated (33%) in a more recent textbook (Boyd 1987). In continuation of that work, we have performed a retrospective analysis of data obtained from all twin placentae from the Birmingham Maternity Hospital in the period 1985 to 1989 inclusive. Zygosity was determined on the basis of placental morphology and blood group analysis where indicated.

Our conclusion was that 38.4% of the twin pairs were monozygous, representing a highly significant increase (χ2= 19.4, P < 0.001) over the results obtained by Cameron (Table 1). It was also apparent, as there was no significant increase in the proportion of dichorionic to monochorionic placentae, that the increase in monozygotic twins has occurred in association with dichorionic placentae. Thus, there appears to have been a genuine and significant increase in the proportion of monozygotic twins in Birmingham over an approximate period of 20 years. This is currently unexplained but in our opinion is not explained by changes in racial composition or fertility treatment.

Table 1.  Zygosity and placental morphology. Values are shown as number n and percentage (%). Total number = 323.
Variablesn (%)
Dizygous166 (51.4)
Definitely monozygous77 (23.8)
Highly likely monozygous47 (14.6)
Total presumed monozygous124 (38.4)
Undetermined33 (10.2)
Dichorionic placentae243 (75.2)
Monochorionic placentae77 (23.8)
Uncertain morphology3(1)

We feel that this is of some interest, not least to the parents of twins, and it is our intention to undertake a prospective joint study with another large maternity unit to investigate this further. The accuracy of genotyping will be improved by use of recently developed DNA analysis techniques which will be carried out in our prospective study.


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