Background: Foetal growth restriction (FGR) refers to a foetus that does not reach its genetically predetermined growth potential. It is well recognised that growth-restricted foetuses are at increased risk of stillbirth, foetal compromise, early neonatal death and neonatal morbidity. Later in life, they are prone to health problems, including increased risk of cardiovascular diseases and neurodevelopmental disorders. Interventions for preventing and treating FGR have been studied in many trials, but evidence is often difficult to synthesise and compare because of differences in the selection and definition of outcomes. To enable future trials to measure similar, meaningful outcomes, we are developing two core outcome sets (COS) - one for prevention and the other for treatment of FGR. Methods: We will review the literature to identify previously reported outcomes. An international panel of relevant stakeholders who have experience of FGR (parent or carer of a baby that was growth restricted, health professional involved in the care of mothers and babies affected by FGR, a person with expertise in FGR research) will rate the importance of each of those outcomes in a series of three sequential online rounds of a Delphi study. Participants will be able to add items to the proposed list in round 1. A final face-to-face consensus meeting will be held with representatives of each stakeholder group at which a final list of outcomes for inclusion in the COS will be agreed. Discussion: The development of COSs in FGR will ensure the collection and reporting of a minimum dataset agreed by stakeholder consensus and will reduce inconsistencies in the reporting of outcomes across relevant trials. Such standardisation in the reporting of outcomes will improve synthesis of evidence and generalisability of knowledge in the future by reducing heterogeneity in outcomes between trials and thus improve the results of systematic reviews and meta-analyses. Ultimately, we hope that the COSs will lead to an improvement in the quality of evidence-based clinical practice, enhance patient care, and improve the quality and consistency of research.