OBJECTIVE: To quantify the effect of different methodological decisions on the identification of potential core outcomes to inform the development of recommendations for future core coutcome set developers. DESIGN: Mixed methods study. SETTING: A core outcome set for pre-eclampsia was used as an exemplar. SAMPLE: A long list of potential core outcomes was developed by undertaking a systematic review of pre-eclampsia trials and performing a thematic analysis of in-depth patient interviews. METHODS: Specific methods used to generate long lists of potential core outcomes were evaluated. RESULTS: Different methodological decisions had a substantial impact on the identification of potential core outcomes. Extracting outcomes from published pre-eclampsia trials was an effective way of identifying 48 maternal, eight fetal, 25 neonatal outcomes, and eight patient-reported outcomes. Limiting the extraction of outcomes to primary outcomes or outcomes commonly reported in pre-eclampsia trials reduced the number and diversity of potential core outcomes identified. Thematic analysis of in-depth patient interviews ensured an additional five patient reported outcomes and six outcomes related to future child health were identified. CONCLUSIONS: Future core outcome set developers should use quantitative and qualitative methods when developing a long list of potential core outcomes. TWEETABLE ABSTRACT: @OfficialNIHR research published in @BJOGtweets informs new recommendations for future @coreoutcomes developers.