Introduction

Randomised controlled trials have greatly improved the quality of evidence guiding clinical practice, but when applied to complex phenomena, they have important limitations. Complex patient populations with poorly quantifiable variations between individuals present one area of difficulty; complex procedures requiring skill and clinical judgment present another. A large, well designed, and well executed randomised controlled trial of breech presentation at term, the “term breech trial,” by Hannah et al rapidly dictated a new standard of care for the management of breech deliveries around the world.1 Yet this trial failed to adequately appreciate both the complex nature of vaginal breech delivery and the complex mix of operator variables necessary for its safe conduct. Widespread acceptance of this trial's results has breached the limits of evidence based medicine.

Hannah et al's trial showed a significant increase in perinatal mortality and morbidity in women randomised to a trial of labour compared with elective caesarean section.1 The trial's methodological flaws have been examined,2^–4 but the intrinsic limitations of applying large scale randomisation to complex phenomena have received little attention. These limitations are the focus of this paper.

Bias of licence

Many of the term breech trial's 121 centres were in North America, where 13% of breech presentations at term were delivered vaginally.5 The study achieved a successful vaginal delivery rate of 57% by asking those centres with vaginal birth rates under 40% in the labour group to increase the rate or withdraw from participation.6 Individual centres rates of vaginal breech delivery at baseline were not reported, but many would have tripled their vaginal delivery rate overnight.

Vaginal breech delivery is a complex procedure …