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BMJ. 2007 April 14; 334(7597): 762.
PMCID: PMC1852002
Perinatal Death in Twins

Author's reply on absolute risk

Gordon C S Smith, professor of obstetrics and gynaecology

In response to Cheetham,1 as stated, our study was able only to address the relative risk to the second twin given the nature of the data.2 We previously addressed both absolute risk and the effect of caesarean section using record linkage of the Scottish Morbidity Record and the Scottish Stillbirth and Infant Death Enquiry. This linkage provides both the numerator and denominator required for the comparisons proposed.

The absolute risk of perinatal death for second twins born at term was 1 in 270 for all causes, 1 in 350 for death due to intrapartum anoxia, and 1 in 500 for anoxic death due to a mechanical cause.3 Planned caesarean section is associated with a lower risk of perinatal death and, if causality is assumed, the number needed to treat is 264 caesarean deliveries to prevent each death.4 These numbers reflect higher absolute risks than calculated by Cheetham. This may reflect flaws in the assumptions of his calculations. It may also reflect the observation that, although the second twin was at increased risk of death at term in both populations, the relative risk was higher in Scotland. We cannot address whether this reflects a greater absolute risk to the second twin in Scotland or to the first twin in England, Northern Ireland, and Wales.

Notes

Competing interests: None declared.

References

1. Cheetham C. Absolute risk: better basis for decision making. BMJ 2007;334:651-2. (31 March.) doi: 10.1136/bmj.39164.420150.1F
2. Smith GCS, Fleming KM, White IR. Birth order of twins and risk of perinatal death related to delivery in England, Northern Ireland, and Wales, 1994-2003: retrospective cohort study. BMJ 2007;334:576-8. (17 March.) [PMC free article] [PubMed]
3. Smith GC, Pell JP, Dobbie R. Birth order, gestational age, and risk of delivery related perinatal death in twins: retrospective cohort study. BMJ 2002;325:1004. [PMC free article] [PubMed]
4. Smith GCS, Shah I, White IR, Pell JP, Dobbie R. Mode of delivery and the risk of delivery-related perinatal death among twins at term: a retrospective cohort study of 8073 births. Br J Obstet Gynaecol 2005;112:1139-44.

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