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Smith et al's conclusion of threefold to fourfold increased mortality in a second twin when born vaginally at term is a relative rather than an absolute risk.1 There was an excess second twin all cause mortality of 73 infants, and an excess second twin anoxic mortality of 60 infants. Altogether there were nearly 100000 births, but the preterm rate (higher in twin births) was not stated, so there might have been 90000 full term confinements. This gives an increased mortality for the second twin of 0.8 per 1000 births, and an increased anoxic mortality for the second twin of 0.67 per 1000 births. If caesarean section successfully avoided all the second twin mortality, 1250 caesarean sections would be needed to save one infant. If caesarean section were only successful in preventing anoxic deaths, 1500 caesarean sections would be needed to save one infant. Although this large number of extra sections might be justifiable, the low possible statistical benefit might persuade some mothers or their obstetricians that a vaginal delivery is a reasonable option for full term confinements. The absolute rather than the relative risk is the correct guide to decision making.
Steer rightly says that the figures do not prove that caesarean sections would prevent deaths in the second twin.2 Fetal behaviour may affect birth order in twin deliveries, and the more vulnerable infant may be delivered second in vaginal but not in caesarean deliveries. The benefit of caesarean section can be proved only by a comparison of total fetal mortality in vaginal and caesarean deliveries.
Competing interests: None declared.