Interactions between opposite-sex dizygotic fetuses and the assumptions of Weinberg difference method epidemiology.

The Weinberg difference method in twin epidemiology may be subject to a major error related to a fundamental assumption that seems not to have been previously questioned. Results presented here indicate that the mortality of like-sex dizygotics (DZs) resembles that of monozygotics (MZs) much more than that of unlike-sex DZs, who are not representative of any other group. Unlike-sex twins enjoy a relative protection from fetal and neonatal mortality and probably from at least one effect of the transient perinatal hypoxia common for second twins. Unlike-sex twins develop, and maintain into adulthood, an intermediate gender phenotype with respect to integration of craniofacial growth.