Method and representation
Most of the included studies described a 2 or 3 round Delphi survey, followed by a face-to-face consensus meeting to finalize the COS. However, some completed studies included only Delphi surveys and one study by Fiala et only undertook a consensus-meeting. The consensus criteria most commonly used for an outcome to be included in the COS was the 70/15 (more than 70% rates the outcome as critically important and less than 15% rates it as not important). The number of outcomes included in the COS ranged between 6 and 48 (Figure3C, Table 1). Only a few studies had less than 10 outcomes in the final COS. None of the ongoing studies mentioned that they had determined or discussed in advance a possible limit to the number of outcomes to be included in the COS to enable implementation and feasibility in research. Two studies described using a “modified nominal group technique” during the consensus meeting in order to reduce outcomes (17, 18).
Researchers were included in all identified studies and healthcare personnel in the majority. Patients were sometimes not included at all in the process (10, 23, 25) or only partly included. Some examples are (8), who used a separate survey consisting of only one round for patients, (21), where patients were included in the Delphi survey, but not in the consensus meeting and (20) where two persons served as proxies for patients . Most of the completed studies involved international participation.
Nine of the studies were assessed as complying to the COS-STAR criteria well in most categories (8, 9, 12-15, 17-19), three showed some deviations (11, 21, 24) and six of the studies were assessed as having major shortcomings in reporting (2, 10, 20, 22, 23, 25-27) table S1. Most of the completed studies lacked information about whether outcomes had been excluded at some stage or if outcomes had been merged. No studies mentioned whether they deviated from the study protocol in any way.

Discussion

Main Findings

Core outcome sets are an agreed standardised collection of outcomes that should be measured and reported for a specific area of health. These sets represent the minimum that should be measured and reported in all clinical trials of a specific condition and are also suitable for use in other types of research and clinical audits. Although there are examples of well-established sets such as Outcome Measures in Rheumatology (OMERACT) for rheumatoid arthritis, they are still relatively rare in most medical fields. The outcomes in the set should represent the minimum to be collected in all trials, but researchers should continue to measure and report additional outcomes of particular relevance to their topic.
This review of pregnancy and childbirth revealed a complete lack of any ongoing or existing COS in the field of mental health, such as postpartum depression, post-traumatic stress disorder after birth, or postpartum psychosis. Consequently, SBU initiated the development of COS for studies of treatment of perinatal depression (2, 27). There are only a few COS on intrapartum care, for such conditions as slow progress in labour, trial of labour after previous caesarean section and postpartum endometritis. One of the topics for which most COS have been compiled is the field of physical conditions and complications during pregnancy.
It is important to consider how many outcomes a COS can include and still be applicable and useful for research. This systematic review discloses that the COS identified range between 6 and 48 outcomes. Only a few of the included finalised COS had less than ten outcomes. None of the identified studies discussed the relationship between the number of outcomes in the COS and the median number of outcomes in the studies for which the COS is intended. Nor did any of the protocols suggest a possible limit to the number of outcomes that might be included in the intended COS. In order to increase the implementation of developed COS, it is important to consider how the number of outcomes included will affect the usefulness of the COS. Some limitation of outcomes might increase the likelihood that the COS will be applied in future research.
It is also important to note that the development of a COS which focuses on what  to measure may need to be followed by decisions about how  and when  to measure these outcomes. Even if the outcomes themselves are consistent across the studies, lack of consistency in how or when outcomes have been measured can undermine efforts by systematic reviewers to compare, contrast and combine the results of multiple studies. Unfortunately, very few of the identified COS mentioned how and when to measure the outcomes in the developed COS.

Strengths and Limitations

Some limitations to the systematic review should be noted. In the systematic review we checked compliance to COS-STAR in the included studies. However, it would have been optimal to be able asses the methodological quality of the included studies using a tool developed for this purpose. We believe that the development of such a tool is desirable and that some of the questions used in this article (Appendix S2) could be helpful. In this systematic review we decided to have an inclusive approach and might have included studies that are not principally intended for research use, but for other purposes, such as clinical follow-up.
This review focuses on maternal health. COS restricted to the infant, which might also be of interest to researchers, were beyond the scope of this review.
A strength of this study is that it is methodologically sound and robust, and all results have continuously been reviewed by experts from the Swedish Agency for Health Technology Assessment and Assessment of Social Services (SBU), as well as by external reviewers. Another strength is the attempt to assess the reporting of the included COS using an assessment tool based on the COS-STAR reporting guide (Appendix S2).
Interpretation
In 2017, Duffy et al published a systematic review of published and ongoing COS related to the health of women and newborns (28). The scope of their paper is somewhat broader, including conditions other than those related to pregnancy and childbirth. In all, they identified four completed COS, of which three were related to obstetric care. In the last two years, a substantial number of COS have been completed and 39 ongoing studies have been identified.
Conclusion
This systematic review discloses an increasing number of COS for pregnancy and childbirth. This is gratifying, hopefully leading to studies which focus on important outcomes and research that is more readily synthesised in systematic reviews, thus increasing evidence in support of interventions. The review reveals that a large number of the ongoing and completed COS studies address physical conditions and complications during pregnancy. There was a lack of COS for delivery. No COS was identified for perinatal mental health. Accordingly, SBU initiated the development of COS for studies of treatment of perinatal depression (2, 27).
Contribution of authorship: Study concept and design: CH, MÖ, AS, MJ, FT. Literature search AJ. Selection of studies and extraction of the relevant information CH and MÖ. Analysis and interpretation of data: CH, MÖ, AS, MJ, FT. Drafting of the manuscript: CH and MÖ. Critical revision of the manuscript for important intellectual content: AS, MJ, FT, CH, MÖ, SF, AJ.
Data availability Data are available on request.
Disclosure of interests: The authors report no conflict of interest, all authors filed a conflicts of interest form used by Swedish governmental agencies before engagement. These are available upon request.
Details of ethics approval: Systematic review, no ethical approval needed.
Funding: The project was conducted within the Swedish Agency for Health Technology Assessment and Assessment of Social Services assignment, external funding was not sought or used.
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