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.

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.​overnight.

Human skill varies. This is particularly evident when tasks are complex, require careful judgment, and have narrow margins for error. Increasingly complex tasks are discriminating, with more effort and skill required to master them. The safe vaginal delivery of a breech baby requires considerable skill and is a discriminating obstetrical procedure. Skill is required in multiple arenas: not just in the delivery technique, but in ultrasound assessment, the selection of cases, intrapartum fetal surveillance, the conduct of labour, and paediatric support. A coordinated, well functioning unit is difficult to quantify.

Randomisation improves the internal validity of trials by homogenising study and control populations thereby avoiding bias from differences between the two. Clinically important factors that are variable within populations are, however, homogenised as well. Their importance to the outcome is lost, and the trial loses external validity for individuals within subsets of the population. Results represent the mean outcome for all participants and are not applicable to subgroups at lower risk. Although subgroup analysis, found in the term breech trial, can potentially identify subpopulations in which a procedure may be safer, it is statistically weak. When phenomena are complex, many patient and practitioner characteristics influence outcome, rendering individual subgroups small and meaningful analysis of them difficult.

Historically, the greatest decrease in perinatal mortality from vaginal breech deliveries was reported by Bracht in 1938.²⁰ Other techniques recommended to enhance the safety of vaginal breech delivery include routine determination of fetal weight, head attitude, and nuchal or presenting cord using ultrasonography; continuous fetal monitoring; radiological pelvimetry; cautious attention to the progress of labour; and preparing for emergency symphysiotomy should fetal parts become trapped.²¹ Although poorly amenable to scientific analysis, some of these techniques are likely to be important for safe vaginal breech delivery. None were included in the term breech trial.

Randomised trials often utilise short term end points because they are easier to measure than longer term outcomes. It is also easier to show a statistical difference in a combined end point rather than a single end point. Yet combined end points can be misleading.²⁴ In the term breech trial, the end point included perinatal mortality and various short term morbidities, including hypotonia, transient brachial plexus injury, and isolated low arterial cord pH or base excess, whose lasting significance is unclear. In countries with low perinatal mortality, this combined end point occurred in 5.7% of planned vaginal deliveries and 0.4% of women undergoing elective caesareans. Mortality was not significantly different (3 of 511 or 0.6%) in the planned vaginal delivery group compared with zero in the planned caesarean group. One of these deaths, included in the intention to treat analysis, occurred before the onset of labour in a cephalic twin weighing 1150 g, highlighting concerns about the adequacy of case selection and raising questions about the appropriateness of intention to treat analysis at all cost. Regardless, the impact of the trial's results was due primarily to an excess of short term morbidity in the planned vaginal delivery group.

Delivering a breech presentation vaginally is a skill: guided by science, its safety relies on the experience of practitioners and caution. In the term breech trial, large scale randomisation, which homogenised both the study population and clinical intervention, resulted in an average level of care in an average population, limiting the trial's external validity in centres showing above average skill and in women of below average risk. Encouraging practitioners to exceed their comfort level ensured a high vaginal delivery rate and adequate statistical power, but introduced a bias of licence and compromised safety. Using a combined short term end point overstated any true effect on long term neurodevelopment.