Composite outcomes that combine a number of individual outcomes (such as types of morbidity) are frequently used as primary outcomes in obstetrical trials. The main argument for their use is to ensure that trials can answer important clinical questions in a timely fashion, without needing huge sample sizes. Arguments against their use are that composite outcomes may be difficult to use and interpret, leading to errors in sample size estimation, possible contradictory trial results, and difficulty in interpreting findings. Such problems may reduce the credibility of the research, and may impact on the implementation of findings. Composite outcomes are an attractive solution to help to overcome the problem of limited available resources for clinical trials. However, future studies should carefully consider both the advantages and disadvantages before using composite outcomes. Rigorous development and reporting of composite outcomes is essential if the research is to be useful.