IntroductionGestational diabetes increases maternal and offspring complications in pregnancy and cardiovascular complications in the long term. The nutritional supplementmyo-inositol may prevent gestational diabetes; however, further evaluation is required, especially in multiethnic high-risk mothers. Our pilot trial onmyo-inositol to prevent gestational diabetes will evaluate trial processes, assess acceptability to mothers and obtain preliminary estimates of effect and cost data prior to a large full-scale trial.Methods and analysisEMmY is a multicentre, placebo-controlled, double-blind, pilot, randomised trial, with qualitative evaluation. We will recruit pregnant women at 12–15⁺⁶weeks’ gestation, with gestational diabetes risk factors, from five maternity units in England between 2018 and 2019. We will randomise 200 women to take either 2 g ofmyo-inositol powder (intervention) or placebo, twice daily until delivery. We will assess rates of recruitment, randomisation, adherence to intervention and follow-up. Gestational diabetes will be diagnosed at 24–28 weeks as per the National Institute for Health and Care Excellence (NICE) criteria (fasting plasma glucose: ≥5.6 mmol/L and 2-hour plasma glucose: ≥7.8 mmol/L). We will assess the effects ofmyo-inositol on glycaemic indices at 28 weeks and on other maternal, fetal and neonatal outcomes at postnatal discharge. Qualitative evaluation will explore the acceptability of the trial and the intervention among women and healthcare professionals. Cost data and health-related quality of life measures will be captured. We will summarise feasibility outcomes using standard methods for proportions and other descriptive statistics, and where appropriate, report point estimates of effect sizes (eg, mean differences and relative risks) and associated 95% CIs.Ethics and disseminationEthical approval was obtained through the London Queen Square Research Ethics Committee (17/LO/1741). Study findings will be submitted for publication in peer-reviewed journals. Newsletters will be made available to participants, healthcare professionals and members of Katie’s Team (a patient and public advisory group) to disseminate.Trial registration numberISRCTN48872100.Protocol version and dateVersion 4.0, 15 January 2018.