(212) Rate of Mosaicism on Diagnostic Testing for Sex Chromosome Abnormalities on NIPT

This retrospective review included patients with positive NIPT for SCA who underwent diagnostic testing between March 2020 and June 2025. Patients with incomplete records, contraindications to NIPT (e.g., organ/bone marrow transplant, hematologic malignancy) and normal NIPT results were excluded. Medical records were reviewed for demographics and karyotype results. The primary outcome was the rate of fetal & placental mosaicism. Fetal mosaicism was defined as mosaicism confirmed on amniocentesis karyotype, while confined placental mosaicism (CPM) was defined as mosaicism on chorionic villus sampling (CVS) with a subsequent normal amniocentesis karyotype. Descriptive analysis were used.

Results:

Ninety-one patients met inclusion criteria and, all underwent diagnostic testing. Of these, 38 cases (41.8%) were confirmed. Among confirmed cases, 26 (68.4%) were non-mosaic and 12 (31.6%) were mosaic. Of the mosaic cases, 7 were fetal mosaic, 4 were CPM, and 1 showed mosaicism on CVS but was not confirmed on amniocentesis. The types of mosaic SCAs included 10 cases of mosaic monosomy X, and 2 cases of mosaic triple X syndrome. CVS was the most common diagnostic method, and 13% of patients who had CVS required follow‑up amniocentesis to clarify mosaicism type.

Conclusion:
Mosaicism accounted for nearly one-third of confirmed SCA cases, with both fetal and placental mosaicism observed. Mosaic monosomy X was the most frequently identified subtype. These findings underscore the critical role of diagnostic testing, including follow-up procedures to differentiate true fetal SCA from confined placental mosaicism and guide appropriate counseling and shared clinical decision-making.