LONG-TERM EFFECT OF PRENATAL HETEROGENEITY AMONG MONOZYGOTES

Twenty pairs of monochorionic and 24 pairs of dichorionic MZ twins wer e studied. The design also included a group of like-sex DZ twins to al low for comparison between MZ and DZ when necessary. Anthropometrical measures were available at birth and at the time of evaluation (8-12 y ears old). The cognitive psychological battery included Vocabulary and Block Design from the WISC-R, the K-ABC, two perception tests and a m ental rotation test. A chorion effect was observed on anthropometrical measures: the monochorionic MZ pairs were less similar than the dicho rionic MZ pairs. A chorion effect was also observed on the Block Desig n but in the reverse direction, confirming a previous report: monochor ionic MZ pairs were more similar than the dichorionic MZ pairs. No cho rion effect was significant for the other psychological variables. The discussion focuses on the magnitude of the chorion effect and the est imation of the sample sizes necessary to achieve good statistical powe r for detecting it, and on the impact of placental. heterogeneity in M Z twins on the estimation of heritability.