SUMMARY The Authors have analyzed the separate and combined influences of zygosity, placentation and sex on intra-pair birth-weight differences in 124 MZ and 124 DZ twin pairs. The highest influence on birth weight was found to be due separately to zygosity and placentation (respectively in cases of dizygosity and of a single placenta). The single placenta causes more circulatory unbalance and, as a consequence, more intrapair weight difference. This finding may help to explain the higher frequency of blighted fetuses in utero in MZ twin pregnancies and that of discordant congenital heart disease in female MZ twins.