A Practical Prenatal Ultrasound Classification System for Lower Limb Anomalies-PRELLIM Classification.

Objective: To address the current lack of a prenatal classification system for fetal lower limb anomalies, we developed and evaluated the PRELLIM (PREnatal Lower LIMb impairment) classification.

Method: A systematic literature review was conducted to identify existing classifications. Based on sonographic features, we developed the PRELLIM classification and applied it to a retrospective cohort of fetuses with isolated lower limb anomalies assessed between 2007 and 2024 at Amsterdam UMC's fetal medicine unit.

Results: No standardized prenatal classification system for lower limb anomalies was found. PRELLIM distinguishes isolated and non-isolated anomalies and categorizes them into clinically relevant subgroups (absent/short, duplication, fusion, contracture, bowing and other). It was applied to 643 fetuses with isolated lower limb anomalies. Contractures were most common (n = 599; 93.2%), followed by poly(syn)dactyly (n = 26; 4.0%), reduction defects (n = 9; 1.5%), bowing (n = 5; 0.8%), and a case of sirenomelia (0.1%). Three additional cases (0.4%) were classified as "other": two lymphangiomas and one amniotic band with lower leg constriction.

Conclusion: PRELLIM is the first prenatal classification tailored to sonographically detectable lower limb anomalies. It aims to enhance diagnostic consistency, improve interdisciplinary communication, and support prenatal counseling and decision-making.