The effects of chorion type on normal and abnormal developmental variation in monozygous twins.

To determine the effects, if any, of chorion type on normal and abnormal developmental variation in monozygous (MZ) twins, we tested the hypothesis that disparate environments that are related to chorion type have no effect on this variation. The parameters studied included congenital anomalies and dermatoglyphics (total ridge count and right-left asymmetry). With the exception of total ridge count, analyses of these data failed to reject the null hypothesis. Dichorionic MZ twins had a significantly greater within-pair variation than monochorionic MZ twins for total ridge count. In summary, then, these data could offer little support to prior speculation that monochorial placenta may present less favorable environments for feta development.