Background: Women with pregnancy complicated by pregestational diabetes experience increased rates of adverse pregnancy outcomes. Prepregnancy care is the targeted support and additional care offered to those women who are planning pregnancy and is associated with improved outcomes. However, there is significant heterogeneity in the outcomes measured and reported in studies evaluating the effects of prepregnancy care, which makes meaningful comparison difficult. The aim of this article is to present a protocol for a study to develop a Core Outcome Set (COS) for trials and other studies evaluating the effectiveness of prepregnancy care for women with pregestational diabetes mellitus.Methods/Design: This study will include a systematic review of the literature to identify outcomes that have previously been reported in studies evaluating prepregnancy care for women with pregestational diabetes. We will then prioritise these outcomes from the perspective of key stakeholders, including women with pregestational diabetes as well as clinicians, using a Delphi survey. A final consensus meeting will be held with stakeholders to review and finalise the outcomes.Discussion: The expectation is that the COS will always be collected and reported in all clinical trials, audits of practice and other forms of research that involve prepregnancy care programs for women with pregestational diabetes. This will facilitate comparing and contrasting of studies and allow for combining of appropriate studies with the ultimate goal of improved patient care.