Pregnancies referred before 28 weeks for isolated reduced fetal size: a 10-year retrospective cohort study

To describe pregnancy and post-natal outcomes, initial phenotypes, and etiologic investigations in singleton pregnancies referred before 28 weeks for isolated reduced fetal size. We conducted a retrospective single-center cohort study at a tertiary referral center from 2016 to 2025. Eligible pregnancies had an estimated fetal weight or abdominal circumference below the 10th percentile before 28 weeks, without major structural anomalies at initial ultrasound. Outcomes were analyzed overall and by three locally defined exploratory categories: presumed placental FGR, genetic diagnosis, and SGA without identified genetic or local placental criteria. Among 176 pregnancies, 19 (10.8%) ended in termination, 13 (7.4%) in intrauterine death, and 144 (81.8%) in live birth, including 12 deaths; long-term follow-up data were available for 41 children. 133 cases (75.6%) were categorized as presumed placental FGR, 16 (9.1%) had a genetic diagnosis, and 27 (15.3%) were categorized as SGA. Presumed placental FGR accounted for most intrauterine deaths and deaths after live birth, whereas 26/27 SGA pregnancies (96.3%) resulted in live birth without documented post-natal death. Live birth without documented post-natal death occurred in 60.0% of pregnancies with estimated fetal weight <1st percentile, 41.7% with oligohydramnios, and 45.5% with abnormal cerebroplacental ratio, versus 89.4%, 80.1%, and 78.9% without these findings. Among 62 pregnancies undergoing amniocentesis, 14 prenatal genetic diagnoses were identified (22.6%). Pregnancies referred before 28 weeks for isolated reduced fetal size have heterogeneous outcomes. Genetic diagnoses were identified among selectively tested pregnancies despite an apparently isolated presentation.