Objective To develop a caesarean birth core information set. Caesareans are the most common surgery performed in many countries. Women need information for informed decision-making and consent. Core information sets (CISs) provide baseline information, agreed upon by parents and clinicians, for discussion before a procedure. Design Two-phase consensus study using modified Delphi. Setting United Kingdom, 2024 Sample People planning a pregnancy/currently pregnant/new parents and maternity professionals Methods Phase 1: Long-list of information points identified from 273 systematic reviews, 50 patient leaflets, three pre-existing qualitative studies, and a stakeholder survey (n=230); Operationalised into a Delphi questionnaire comprising 11 information points with 108 items. Phase 2: Think-aloud interviews (n=9) informed questionnaire restructure into information about planned caesarean birth, unplanned caesarean birth (within 72 hours), and emergency caesarean birth (EMCB; ≤30 minutes), followed by two-round Delphi survey and consensus meetings. Results N=360 participated in the Delphi survey Round 1. All items were carried forward, and three were added for Round 2 (n=188/56.4% attrition rate). From Round 2, one item was removed, 73 included, and 37 items no-consensus. Free-text responses identified an unmet need for a postnatal EMCB-CIS. Over four meetings (n=36) consensus was reached for an antenatal-caesarean-birth-CIS (14 points), EMCB-CIS (5 points), and a postnatal EMCB-CIS (12 points). Conclusions This study has established three caesarean birth CISs to support informed decision-making discussions between women and clinicians: (1) CIS for planned and unplanned caesareans when there is time for discussion; (2) CIS for EMCB (within 30 minutes); (3) CIS post-EMCB pre-hospital discharge.