Strengths and limitations
The main strength of this study is that all pregnancies included were fully evaluated and delivered in a single tertiary center, with a long tradition of vaginal delivery and breech extraction of the second twin.
To our knowledge, this is the first study considering the mode of onset of labor as well as 2-years-neurodevelopment. Although the study population is not large, we were able to define more detailed and accurate perinatal outcome measures than it is possible to do with registry-based data. Furthermore, this is a primary analysis, and not a subgroup analysis of a previous larger study.
Though retrospective studies are limited by confounders, it is not possible to develop this study as a randomized controlled trial for ethical reasons. In our center, at least two obstetricians and two neonatologists were always present at birth, and an operating room was available for emergency CS if required; therefore, the results can be extrapolated to centers that meet the same conditions.