Expert consensus on the morphological characteristics diagnostic criteria and genetic counseling of human chromosomal polymorphisms

Karyotype analysis is a routine prenatal diagnostic procedure in genetic diagnosis and counseling. Chromosomal polymorphisms are frequently observed in karyotyping. However, the absence of standardized criteria for defining polymorphisms has led to inconsistencies across laboratories in determining whether a specific chromosomal variant should be classified as a polymorphism and how it should be symbolized. This lack of consensus hinders the mutual recognition of karyotype reports and complicates the clinical interpretation of polymorphisms. In this expert consensus, we collected cases confirmed of various polymorphic karyotypes, examined their morphological features under different banding techniques, and compared the characteristics of G-, C-, and N-bands across polymorphism types. Based on these comparisons, we established defining features of polymorphic G-bands and categorized the polymorphisms in accordance with the International System for Human Cytogenomic Nomenclature (ISCN 2024). We further propose criteria for the identification, stepwise diagnostic procedures, models for informed consent, and frameworks for clinical interpretation. This consensus aims to standardize the recognition of chromosomal polymorphisms, facilitate the standardization of karyotype reports, promote consistency in genetic counseling, and address long-standing challenges in the field—such as the lack of uniform standards for polymorphism classification and divergent interpretations of the same variant in genetic counseling.