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Sir,

We read the aims of the Twins Timing of Birth Randomised Trial (TTBRT) with interest, its methodology and results with concern, and its conclusions with dismay.1 According to the power calculations, this multicentre randomised controlled trial over 7½ years in 13 centres required 460 women to reduce an estimate of composite serious adverse outcome (death or any of 17 outcome measures) to that of singleton infants at the same gestational age, but was concluded after recruitment of 235 women through lack of resources.

A recent opinion piece referenced a number of well-publicised randomised controlled trials and warned of the danger of composite perinatal outcomes.2 Although their use to appropriately power a study is understandable, the relevance of each of these outcomes needs to be comparable, unless they are statistically weighted. In the TTBRT the ‘serious adverse outcomes’ in the standard care group were significantly swayed by 24/29 pregnancies involving a birthweight below the third centile on a singleton chart, with only eight other adverse outcomes in each group. Data on the 3–10 centile weight range was not provided. Once all of the other adverse outcomes were excluded, being small for gestational age could not be considered serious in its own right, and may reflect in fact a failure of monitoring and appropriate surveillance in this group. For singletons at least, there is evidence that being ‘small’ on population statistics only modestly increases the risk of poor perinatal outcome, unless a fetus is also small by customised charts.3 It is misleading for the authors to state that the reduction in adverse outcomes at term is ‘predominantly associated...less than the third centile’ when in fact the authors should be stating ‘almost entirely’ instead of ‘predominantly’.

We are most surprised to see the publication of a study grouping management of monochorionic and dichorionic twin pregnancies together, with no regard to their (patho)physiological differences. An editorial published during this trial has highlighted the fact that these different twin types carry different risks and require different management protocols,4 so their grouping for research is ill-advised. Contemporary clinical management advocates earlier delivery for monochorionic diamniotic twin pregnancies (generally around 35–36 weeks gestation to minimise late fetal loss). The authors recognise in their discussion the differing mortality and morbidity between twin types, but chose to ignore this in their work and it may have been fortunate that there were not more perinatal losses in the 40 monochorionic diamniotic pairs in this trial.

The authors acknowledge that their trial was ‘relatively underpowered to assess our primary outcome...’ yet paradoxically state that ‘the findings... support recommendations for women with an uncomplicated twin pregnancy to birth at 37 weeks of gestation’, which we feel is incorrect. In fact this study shows that elective delivery at 37 weeks prevents infants from being classified as small for gestational age (using singleton charts only) with no change in neonatal morbidity, and that earlier delivery for dichorionic diamniotic twins may not increase maternal morbidity. We are most concerned that unquestioning acceptance and injudicious application of this trial as published may have adverse effects on the management of twin pregnancy. The authors should withdraw the conclusion.

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