Information about midwifery, midwives, and Grateful Birth

From the Third Edition: Obstetrics Normal and Problem Pregnancies Edited by Steven G. Gabbe – Jennifer R. Niebyl – Joe Leigh Simpson

Because monochorial placentas can only occur in monozygotic (MZ) pregnancies, study of the membranes will establish zygosity in 20 percent of the cases in the United States. In approximately 35 percent of the cases the twins will be of opposite sex and therefore necessarily dizygotic (DZ). This Leaves only 45 percent of cases (twins of like sex having dichorionic placentas) in which further studies are necessary to determine zygosity.

Cameron(6) found that this 45 percent breaks down into 8 percent MZ and 37 percent DZ. He states that “the percentage of monozygotic dichorionic pairs is inversely proportional to the completeness of genotyping”. In other words, the more genotypic markers that are studied, the greater the likelihood of demonstrating dizygosity. At present, the most accurate method for determination of twin zygosity is by analysis of DNA polymorphism.

References 6. Cameron AH: The Birmingham Twin Survey. Proc P. Soc Med 61:229, 1968

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Last Updated: October 3, 2015