Birthweight was measured on 188 monochorionic monozygotic, 54 dichorionic monozygotic, 102 like-sexed dizygotic, and 94 unlike-sexed dizygotic liveborn twin pairs. Overall, males were found to be significantly heavier than females. These differences were not significant, however, when birthweights were compared within zygosity/chorion-type categories. Males were also characterized by a slightly greater overall total variance. Comparisons of intrapair variation of monochorionic and dichorionic monozygotic twins revealed significant differences between monochorionic pairs and dichorionic separate pairs and no significant differences between monochorionic pairs and dichorionic fused pairs. The results of this study suggest that placental proximity may have as important an influence on variation in birthweight as does the presence or absence of vascular anastomoses.Key words: Birthweight, Twin pregnancy, Placental effects, Length of gestation, Sex effects, Fetal hormones Variation in human birthweight is known to be influenced by such factors as length of gestation [ 3 , 6 ] , race [ 1 ] , sex [17], parity [9] and maternal smoking history [11]. In addition, "maternal factors" are recognized to have powerful and possibly overriding effects on the expression of this trait [10,13], but the relative importance of fetal genoThis is paper No.