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Zygosity, placental membranes and Weinberg's rule in a Danish consecutive twin series.

The usual assumption that monochorionic twins are monozygotic has recently been questioned, based on blood group discordance in 3 of 12 monochorionic pairs. Therefore, this study evaluates the validity of zygosity diagnosis based on examination of placental membranes, and at the same time evaluates Weinberg's differential rule in a Danish consecutive twin series. All twin pairs, 265 like-sexed and 87 unlike-sexed pairs, born at Odense University Hospital, Denmark, from 1 January 1980 through 31 August 1988, were ascertained. In 229 like-sexed pairs, zygosity was determined by serological methods, and in 190 of these, the fetal membranes could be reliably classified. Fifty-nine monochorionic twin pairs were identified and none of those pairs were discordant for any genetic markers. Therefore, we conclude that monochorionic twins can be considered monozygotic. The distribution of monozygotic and dizygotic pairs was in accordance with Weinberg's rule.