BIRTH-WEIGHT IN OPPOSITE SEX TWINS AS COMPARED TO SAME-SEX DIZYGOTIC TWINS

The question addressed in the present report is whether the large birt h weight differences in dizygotic twin pairs of opposite sex (DZos), e specially in 'male first' couples - observed by Blickstein and Weissma n (Blickstein 1, Weissman A. Birth weight discordancy in male-first an d female-first pairs of unlike-sexed twins. Am J Obstet Gynecol 1990;1 62:661-663) and replicated in the present study - can be explained by two general influences on birth weight, viz. sex and birth order, or w hether some specific effect on fetal growth has to be assumed that is present only in twin pairs of differing sex. The associated enhanced h ealth risk would hit the female twin (from a male first-female second couple) in the first place. If the hypothesis is correct, then one may expect that birth weight of twins is somehow dependent on the sex of the co-twin. This was studied in 3069 twin pairs born in The Netherlan ds since the end of 1986. Results show that among DZ twins, birth weig ht is not affected by the sex of the co-twin. Therefore, birth weight differences in DZos pairs have to be ascribed to the general effects o f sex and birth order. There is no effect that is specific to DZos pai rs only.