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Estimation of the Stillbirth Rate in Twin Pairs According to Zygosity

Published online by Cambridge University Press:  21 February 2012

Johan Fellman*
Affiliation:
Folkhälsan Institute of Genetics, Population Genetics Unit, Helsinki, Finland. [email protected]
Aldur W. Eriksson
Affiliation:
Folkhälsan Institute of Genetics, Population Genetics Unit, Helsinki, Finland.
*
*Address for correspondence: Johan Fellman, Folkhälsan Institute of Genetics, Population Genetics Unit, POB 211, FIN-00251 Helsinki, Finland.

Abstract

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It is common opinion that the stillbirth rate is higher among monozygotic (MZ) than among dizygotic (DZ) twins. This is supported by the fact that stillbirth rates are higher among same-sexed than among opposite-sexed twins, and the relatively high stillbirth rates among twins of young mothers. In this study we present a method to estimate the stillbirth rates for MZ and DZ twins and identify the difference. We performed analyses based on the assumptions of (a) Weinberg's differential rule, including the assumption that the secondary sex ratio is 100, (b) the stillbirth rates among opposite-sexed twins hold for all DZ twins, and (c) the stillbirth rates estimated for MZ and DZ male and female twins yield for both sexes the observed total number of stillborn twins. Our methods are applied to data from Sweden, 1869–1967, the Åland Islands, 1750–1949, Saxony, 1881–1900, and England and Wales, 1996–2003. We observed that the ratio between the estimated stillbirth rates among MZ and DZ twins were on average 1.75, and the ratio among same-sexed and DZ (opposite-sexed) twins were on average 1.31. For Sweden and Saxony similar values were obtained, but for England and Wales the values were higher and for Åland lower. With exception of Åland, the estimated stillbirth rates were in all populations the lowest for DZ, medium for same-sexed and highest for MZ twins.

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Articles
Copyright
Copyright © Cambridge University Press 2007