Antenatal discovery of mosaic trisomy 14 in an early-onset malformative syndrome

Trisomy 14 mosaicism is a rare chromosomal disorder caused by the presence of an extra chromosome 14 in a subset of cells. Its true prevalence is unclear due to frequent early pregnancy loss. Clinical manifestations vary widely and include fetal growth restriction, craniofacial dysmorphia, cardiac and genitourinary anomalies, with no consistent correlation between mosaicism level and severity. We report a case of prenatal trisomy 14 mosaicism diagnosed via chorionic villus sampling and confirmed in fetal tissue. Ultrasound at 15 weeks showed multiple congenital anomalies affecting the craniofacial region, heart, and limbs , along with growth restriction. Given the severity, termination of pregnancy was performed at 17 weeks. Postmortem examination confirmed the anomalies, including abnormal male external genitalia. FISH analysis on muscle tissue demonstrated trisomy 14 mosaicism in approximately 35% of nuclei. This case contributes to the limited prenatal literature on trisomy 14 mosaicism and emphasizes the value of early imaging and cytogenetic analysis in diagnosis and counselling. It highlights the importance of concurrent detailed early sonographic evaluation and targeted cytogenetic testing in suspected malformative syndromes, as well as the diagnostic value of postmortem tissue-specific FISH to better characterize the mosaic distribution. Because mosaicism levels may vary markedly between tissues and do not necessarily correlate with phenotypic severity, this case also illustrates the limitations of chorionic villus sampling and amniocentesis, and supports multi-tissue sampling in fetopathological assessment to refine prognostic evaluation and genetic counselling.