INTRODUCTION: Fetal growth restriction (FGR) affects 5%-10% of all pregnancies, contributing to 30%-50% of stillbirths. Unfortunately, growth restriction often is not detected antenatally. The last weeks of pregnancy are critical for preventing stillbirth among babies with FGR because there is a pronounced increase in stillbirths among growth-restricted fetuses after 37 weeks of pregnancy. Here we present a protocol (V.1, 23 May 2016) for the RATIO37 trial, which evaluates an integrated strategy for accurately selecting at-risk fetuses for delivery at term. The protocol is based on the combination of fetal biometry and cerebroplacental ratio (CPR). The primary objective is to reduce stillbirth rates. The secondary aims are to detect low birth weights and adverse perinatal outcomes. METHODS AND ANALYSIS: The study is designed as multicentre (Spain, Chile, Mexico,Czech Republic and Israel), open-label, randomised trial with parallel groups. Singleton pregnancies will be invited to participate after routine second-trimester ultrasound scan (19+0-22+6 weeks of gestation), and participants will be randomly allocated to receive revealed or concealed CPR evaluation. Then, a routine ultrasound and Doppler scan will be performed at 36+0-37+6 weeks. Sociodemographic and clinical data will be collected at enrolment. Ultrasound and Doppler variables will be recorded at 36+0-37+6 weeks of pregnancy. Perinatal outcomes will be recorded after delivery. Univariate (with estimated effect size and its 95% CI) and multivariate (mixed-effects logistic regression) comparisons between groups will be performed. ETHICS AND DISSEMINATION: The study will be conducted in accordance with the principles of Good Clinical Practice. This study was accepted by the Clinical Research Ethics Committee of Hospital Clinic Barcelona on 23May 2016. Subsequent approval by individual ethical committees and competent authorities was granted. The study results will be published in peer-reviewed journals and disseminated at international conferences.