The observation that, in twin pregnancies, perinatal mortality is
about 40% lower after assisted conception compared with natural
conception, needs to be tempered with caution. The authors accept that
chorionicity plays a part because dichorionic fare better than
monochorionic pregnancies and monochorionic comprise 5-7% of assisted
compared with 30% of natural twin pregnancies. However, the effect of
chorionicity is dismissed because of the lack of effect in studies that
controlled for zygosity. That zygosity does not matter needs to be better
documented. In an analysis of national England and Wales data for 1993-5,
the relative risk (RR) for monozygous (MZ) compared with dizygous (DZ)
twins for both twins to be stillbirths was 10.9 (95% confidence interval
(CI) 6.9 to 17.1; p<0.00001), for stillbirth/infant death twins the RR
was 3.8 (95% CI 2.4 to 6.1; p<0.00001), for one stillbirth/one survivor
the RR was 1.3 (95% CI 1.1 to 1.5; p=0.005) and for both to be livebirths
and both to die in infancy the RR was 2.3 (95% CI 1.9 to 2.8;
p<0.0001). Reference 1.
An additional bias associated with zygosity is that there is a highly
significant inverse trend in proportion of MZ twins with decreasing
birthweight. MZ comprise about 50% of twins of birthweight less than 500g
but only about 25% of those with birthweight >=3000g. Reference 2.
As a partial proxy for excluding MZ, and therefore, monochorionic
twins, so as to compare like with like, it may be worth comparing
perinatal mortality in assisted conceptions with unlike sex twins in
spontaneous conceptions.
The very high risk of intra-uterine death, early (and late) infant
death and serious morbidity in survivors that is associated with
monochorionicity must be adequately accounted for in a comparison of
assisted and spontaneous conceptions.
P.O.D.Pharoah
References
1. Pharoah POD. Neurological outcome in twins. Seminars in
Neonatology 2002;7:223-230.
2. Pharoah POD. Cerebral palsy in the surviving twin associated with
infant death of the co-twin. Archives of Disease in Childhood Fetal and
Neonatal Edition 2001;84:F111-F116.
Competing interests:
None declared
Competing interests:
No competing interests
30 January 2004
Peter O Pharoah
Emeritus Professor of Public Health
Department of Public Health, University of Liverpool, Liverpool L69 3GB
Rapid Response:
Zygosity does matter
The observation that, in twin pregnancies, perinatal mortality is
about 40% lower after assisted conception compared with natural
conception, needs to be tempered with caution. The authors accept that
chorionicity plays a part because dichorionic fare better than
monochorionic pregnancies and monochorionic comprise 5-7% of assisted
compared with 30% of natural twin pregnancies. However, the effect of
chorionicity is dismissed because of the lack of effect in studies that
controlled for zygosity. That zygosity does not matter needs to be better
documented. In an analysis of national England and Wales data for 1993-5,
the relative risk (RR) for monozygous (MZ) compared with dizygous (DZ)
twins for both twins to be stillbirths was 10.9 (95% confidence interval
(CI) 6.9 to 17.1; p<0.00001), for stillbirth/infant death twins the RR
was 3.8 (95% CI 2.4 to 6.1; p<0.00001), for one stillbirth/one survivor
the RR was 1.3 (95% CI 1.1 to 1.5; p=0.005) and for both to be livebirths
and both to die in infancy the RR was 2.3 (95% CI 1.9 to 2.8;
p<0.0001). Reference 1.
An additional bias associated with zygosity is that there is a highly
significant inverse trend in proportion of MZ twins with decreasing
birthweight. MZ comprise about 50% of twins of birthweight less than 500g
but only about 25% of those with birthweight >=3000g. Reference 2.
As a partial proxy for excluding MZ, and therefore, monochorionic
twins, so as to compare like with like, it may be worth comparing
perinatal mortality in assisted conceptions with unlike sex twins in
spontaneous conceptions.
The very high risk of intra-uterine death, early (and late) infant
death and serious morbidity in survivors that is associated with
monochorionicity must be adequately accounted for in a comparison of
assisted and spontaneous conceptions.
P.O.D.Pharoah
References
1. Pharoah POD. Neurological outcome in twins. Seminars in
Neonatology 2002;7:223-230.
2. Pharoah POD. Cerebral palsy in the surviving twin associated with
infant death of the co-twin. Archives of Disease in Childhood Fetal and
Neonatal Edition 2001;84:F111-F116.
Competing interests:
None declared
Competing interests: No competing interests