Estimation of the stillbirth rate in twin pairs according to zygosity.

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 Aland 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 Aland lower. With exception of Aland, the estimated stillbirth rates were in all populations the lowest for DZ, medium for same-sexed and highest for MZ twins.