Blood vessels of the twin placenta in relation to zygosity.

The anatomical characteristics of the arteries, veins and arteriovenous relationships, velamentous vessels, and vascular communications in 148 placentae from multiple pregnancies were studied. All features were found to show discordance of varying degrees, irrespective of the twins' zygosity. Singular involvement of one component of the twin placenta by hydramnios or congenital defects, incidence of vascular communications and the anatomical characteristics of the vessels in conjoined twins, acardiac monsters and triplets, and a chromosomal discordance in a MZ pair, lend additional support to the initial surmise of anatomical characteristics of fetal blood vessels of the placenta being determined by functional demands. It is suggested that the inequalities of prenatal environment be assessed by an examination of the fetal blood vessels of the placenta before drawing homologies in the twin concordance studies.