FSID Unit of Perinatal and Paediatic Epidemiology, Department
of Public Health, Muspratt Laboratory, Liverpool L69 3GB, UK
Correspondence to:
Dr P O Pharoah, FSID Unit of Perinatal and Paediatic Epidemiology, Department of
Public Health, Muspratt Laboratory, Liverpool L69 3GB, UK; p.o.d.pharoah@liv.ac.uk
Background: Twins compared to singletons are at increased
riskof sudden infant death syndrome (SIDS).
Aims: To compare the epidemiology of SIDS in twins and singletonsand to test the hypothesis that monozygous (MZ) were at greater
risk of SIDS than dizygous (DZ) twins.
Methods: Data from the Office for National Statistics on allregistered live births and infant deaths with registered cause
of death "sudden unexpected death in infancy" in England andWales
from 1993 to 1998 were obtained, together with the registeredbirth
weight and, for twins, whether they were of like or unlikesex.
Results: The crude relative risk of SIDS in twins is twice thatin singletons. There has been a significant temporal declinein
SIDS mortality. There is also a significant increase in riskwith
decreasing birth weight for both twins and singletons.The birth
weight specific risk of SIDS in all except for those3000 g is greater in singletons than in
twins. There is no significantdifference in risk of SIDS in like
compared with unlike sextwins.
Conclusions: In spite of a lower risk of SIDS in twins comparedwith singletons for each birth weight group <3000 g, one
component of the higher crude relative risk of SIDS in twinsis
attributable to the higher proportion of twins that are oflow birth
weight. A second component is the higher risk in twinscompared with
singletons for those of birth weight 3000
g. Likesex are at no greater risk than unlike sex twins, which
suggeststhat zygosity is not a significant factor in SIDS.
Keywords: SIDS, twins
Abbreviations: DZ, dizygous; MZ, monozygous; ONS, Office for
National Statistics; SIDS, sudden infant death syndrome
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