Background <p>Pulmonary atresia or critical pulmonary stenosis with intact ventricular septum (PA/CS-IVS) is a rare and heterogeneous congenital heart disease with highly variable postnatal outcomes. Existing prenatal prognostic models are limited by small sample sizes, heterogeneity in echocardiographic variables, and suboptimal validation. Robust prenatal prediction of postnatal circulation is essential for parental counselling, perinatal planning, and selection of candidates for potential fetal intervention.</p> Methods <p>This international observational cohort study will include non-intervened fetuses with a prenatal diagnosis of PA/CS-IVS diagnosed between 16 + 0 and 28 + 6&#xa0;weeks of gestation. Retrospective cases (from 2013 onwards) will be used for model development, and prospectively recruited cases for external validation. Standardized prenatal and postnatal echocardiographic data will be collected using predefined protocols and reviewed by a central core laboratory. Multivariable logistic regression models will be developed following TRIPOD recommendations, with internal validation by bootstrapping. Secondary analyses will explore longitudinal prenatal data and machine-learning approaches.</p> Discussion <p>This study aims to develop and validate a clinically applicable prognostic model for PA/CS-IVS using standardised, multicentre data. The results are expected to improve prenatal counselling and perinatal management, and will support the design of future interventional trials.</p> Trial registration <p>ClinicalTrials.gov, NCT07095829.</p>