Stillbirths in Multiple Births: Test of Independence

Abstract

The stillbirth rate in twins is a more sensitive indicator of environmental hazards than the stillbirth rate in singletons. Medical care or other socioeconomic factors may be more influential for perinatal survival in twin than in single deliveries. Studies have indicated that stillbirths among children in a set of multiple maternities are not independent. Models were considered assuming independent outcomes within a set of multiple maternities. Analyses of the stillbirth rates confirm that the risk of stillbirth among males is almost constantly higher than among females. Any model introduced should assume different stillbirth rates for males and females. The models were tested with both maximum likelihood and minimum chi2 methods. Data was analyzed from Sweden, the Aland Islands, Saxony, England and Wales, and significant discrepancies obtained from the independence models. The same-sexed twin data contain both monozygotic and dizygotic twin sets with apparently different stillbirth rates. Consequently, for same-sexed twins the proposed model could be considered too simple. After improvement by splitting the same-sexed data into monozygotic and dizygotic twin sets, the dependence still remains. The proportion of both same-sexed and opposite-sexed twin pairs that contain two stillborn is greater than what the stillbirth rates and the independence should indicate. Consequently, stillbirth rate estimates based on the relative frequency of twin sets with two stillborn children have a positive bias. When the stillbirth rate decreases, the number of sets with two stillborn children decreases more slowly than would be indicated by independence.

Estimation of the Stillbirth Rate in Twin Pairs According to Zygosity

“…The relative decrease varies between 53 and 68%, which can be compared with 64% among singletons. The rather constant relative decreases noted in Fellman and Eriksson (2006b) and Eriksson and Fellman (2006) seem to hold also for MZ twins. In Table 3 and Figure 2 we present for Sweden the ratio between the SBRs among MZ and DZ twins and the ratio between the SBRs among SS and DZ twins.…”

“…Statistics Sweden published similar information for the period . Swedish data were also considered by Fellman and Eriksson (2006a, 2006b, 2006c and Eriksson and Fellman (2006).…”

“…Eriksson and Fellman (2006) also included influential factors, such as marital status, rural and urban regions and sex combination of the multiple births. The analyses were based on Swedish data for the period 1869-1967. Fellman and Eriksson (2006b studied how reliable WDR is.…”

“…It is commonly accepted that the stillbirth rate (SBR) is higher among monozygotic (MZ) than among dizygotic (DZ) twins (Derom & Derom, 2005;Fellman & Eriksson, 2006b;Lommatzsch, 1902;Machin, 2001;Wedervang, 1924, and for review see Campbell & MacGillivray, 1988). This is supported by the fact that SBRs are higher among same-sexed (SS) than among opposite-sexed (OS) twins or higher multiple births.…”

“…In this study we present a method to estimate SBRs for MZ and DZ twins and identify the differences. In a recent paper, Fellman and Eriksson (2006b) analyze the effect of the dependence between outcomes in multiple maternities. As a preliminary result we can state that zygosity as such cannot wholly explain that dependence.…”