BackgroundDiabetes mellitus is a global health problem and one of the most common medical conditions in pregnancy. A wide range of modifiable risk factors are associated with diabetes mellitus in pregnancy, and it is widely acknowledged that preconception care (PCC) is beneficial for women with pre-existing diabetes mellitus. However, uptake of PCC services is low.ObjectivesTo systematically review qualitative research on PCC for women with pre-existing diabetes mellitus of childbearing age, identify facilitators of and barriers to uptake of PCC and establish themes and gaps in knowledge. Through qualitative interviews explore views on the provision of, and facilitators of and barriers to the uptake of, PCC.DesignMixed methods encompassing a systematic review and qualitative interviews.SettingTwo secondary care sites and 11 primary care sites.ParticipantsWomen of childbearing age with pre-existing type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM) of white British or Pakistani origin.InterventionsNone.AnalysisA narrative synthesis of the literature using thematic analysis and a thematic analysis of the qualitative interview data using the method of constant comparison.ResultsEighteen qualitative studies were included in the systematic review and a quality appraisal was carried out using relevant criteria for qualitative research appraisal, including a narrative summary of study quality. Twelve interviews with women with pre-existing T1DM or T2DM were carried out. This fell short of the original aim of interviewing 48 women owing to challenges in recruitment, especially in primary care. A synthesis of these data shows that uptake of PCC is influenced by a range of factors, including the complexity of pregnancy planning, the skill and expertise of health professionals who provide care to women with diabetes mellitus, the role of health professionals in the delivery of PCC, and the quality of relationships between women and health professionals.LimitationsOwing to significant challenges with recruitment of participants, particularly in primary care, 12 interviews with women with pre-existing T1DM or T2DM were carried out, which fell short of the a priori sample size.ConclusionsReconceptualising PCC to place greater emphasis on pregnancy planning, fertility and contraception would lower some of the existing barriers to uptake of care. It is important to clarify who is responsible for the delivery of PCC to women with pre-existing diabetes mellitus and to ensure that the correct expertise is available so that opportunities for advice giving are maximised. Relationships between women and health professionals should be based on a partnership approach that encourages mutual trust and respect, focusing on positive change rather than negative outcomes.Future workFurther research is needed to investigate the views and experiences of stakeholders that commission, design and deliver PCC services for women with pre-existing diabetes mellitus; to explore experiences of women from minority or ethnically diverse backgrounds; to investigate the role of family support in contraception, pregnancy planning and PCC; and to investigate the management of diabetes mellitus in neonatal care and its role in breastfeeding.Study registrationThis study is registered as PROSPERO CRD42014015592 and ISRCTN12983949.FundingThe National Institute for Health Research Health Technology Assessment programme.