Patient involvement:
A patient representative (FT) was included in the project management group in order to ensure patient input into all aspects of the work.

Results

Eligible studies

The literature search yielded a total of 2699 citations: after review of the abstracts, 138 were assessed in full. Eighty studies which did not meet the inclusion criteria were excluded, leaving 58 relevant studies. Of these, 19 were completed studies with prioritized outcomes and 39 were COS protocols, where the final COS was not yet published (Figure 2). For detailed information about the included studies, see table S1. Excluded studies and the reason for exclusion can be found in table S2. Of the 39 identified ongoing COS studies, the full protocol was identified for 10. The ongoing studies are described in table S3

Published core outcome sets

Of the completed studies, 12 had an expressed intention to develop a COS (8-19). In the remaining seven studies, outcomes were also prioritized, but the main aim of the studies varied somewhat (20-26). The primary aim of two articles was to prioritize future research questions, and this included prioritizing the outcomes to be assessed (20, 26). Two other articles investigated which outcomes should be prioritized in a composite outcome, while other studies considered which outcomes should be assessed in clinical follow-up of patients (21-25). A brief overview of the included completed studies is presented in Table 1.
Of the completed studies, all were published after 2007 and 52% were published during 2018 and 2019 (Figure 3A). The large number of ongoing COS projects identified also indicates a high degree of activity in the field.
Sorting the studies into categories (figure 3B), discloses that most COS, both completed and ongoing, focus on pregnancy and pregnancy-related complications and conditions. There are surprisingly few COS for labour and delivery and physical conditions associated with giving birth. We could, for example, not find any COS with a focus on vaginal delivery or caesarean section delivery as a whole or COS with a focus on interventions related to the actual delivery process. All the identified COS concerned physical health conditions, no COS where identified with respect to mental health during pregnancy or after childbirth.