Cytogenetic and Molecular Findings in Hydrops‐Related Mirror Syndrome

ABSTRACT Objective Mirror syndrome is a rare, life‐threatening condition in which maternal fluid overload mirrors fetal hydrops. Data on genetic findings in affected pregnancies are limited. We compared genetic diagnoses in hydrops cases with and without mirror syndrome. Methods We conducted a retrospective cohort study (2012–2023) of pregnancies with non‐immune hydrops at a single fetal center. Clinical data along with prenatal and postnatal genetic testing results were analyzed. Mirror syndrome was defined as fetal hydrops with new or worsening hypertension and significant maternal edema. Genetic testing rates and diagnostic yields were compared between the groups. Results Of the 192 cases, 140 (73%) underwent genetic testing. Testing rates were similar between mirror (24/31, 77%) and non‐mirror cases (116/161, 72%; p = 0.54). Abnormal findings explaining the phenotype were identified in 29% of mirror and 32% of non‐mirror cases ( p = 0.79). Rates of single‐gene disorders, aneuploidy, and copy‐number variants did not differ ( p > 0.05). Most single‐gene diagnoses involve cardiovascular or lymphatic pathways. Conclusion The rates and types of genetic diagnoses were similar in hydrops cases with and without mirror syndrome. Despite limited power to detect subtle differences, these findings support a similar approach to genetic counseling and evaluation for hydrops cases with and without mirror syndrome.