Do placentation issues bias twin studies?

Abstract

Because monozygous twins are almost genetically identical whereas dizygous twins are genetically no more similar than siblings, greater similarity between members of a monozygotic twin pair than between a dizygotic pair is usually taken as evidence that the condition or trait being studied has a genetic contribution. However, besides greater similarity of genes, monozygous pairs may share more similar environments than do dizygotic pairs. This environmental sharing may occur in fetal life where close placental relationships lead to more similar fetal environments. It may also occur postnatally. If the twins are of the same sex and look alike, the parents may dress, feed and in other ways treat the twins more similarly. Whereas the effects of a more similar postnatal environment have been explored by comparisons of twins raised together or separately, remarkably little is known about the influence of prenatal exposures.