Introduction
The complexity of prenatal diagnosis is underscored by the importance of early detection. As such, standard prenatal care in Ontario, Canada includes universal first trimester screening, with the option of noninvasive prenatal testing (NIPT) of cell-free fetal DNA (cffDNA) as well as optional reflexing to invasive diagnostic testing via chorionic villi sampling (CVS) or amniocentesis by QF-PCR and chromosomal microarray (CMA). In this context, fetoplacental discrepancies present a rare, but significant diagnostic challenge. Occurring in approximately 1-2% of pregnancies, fetoplacental discrepancies are typically uncovered by a false-positive NIPT result due to the presence of abnormal cell lines confined to the placenta, or confined placental mosaicism (CPM), masking a normal fetal karyotype. Here we report two exceedingly rare cases of CPM with non-mosaic fetal abnormalities: Case 1 involves a false negative result obtained via CVS, while Case 2 presented with discordant abnormal findings detected by CVS and amniocentesis.

Case Presentation
Case 1: A prenatal diagnosis was initiated following detection of increased nuchal translucency, absent nasal bone, and tricuspid regurgitation, but a negative NIPT result.



Case 2: A prenatal diagnosis was initiated following detection of increased nuchal translucency and a positive NIPT indicating trisomy 21 (T21), though with a lower-than-expected positive predictive value.

Diagnostic Workup
Case 1: Direct CVS yielded a normal female result by both QF-PCR and CMA. However, short long bones were later identified by ultrasound in the third trimester. Given late gestational age and the nonspecificity of the ultrasound findings, cultured CVS was sent for exome sequencing (ES). Contrary to the initial CMA result from the direct sample, ES revealed mosaic T21. Post-termination genotyping and CMA on cultured CVS corroborated the mosaic T21 findings, and the same analyses on banked umbilical cord DNA returned full T21. The discrepancy between the direct and cultured CVS results may stem from low-level placental mosaicism beyond the limit of detection for CMA, a discrepancy in growth advantage between the normal and abnormal cell lines, or sampling bias with preferential inclusion of cytotrophoblastic cells, presumably containing higher proportions of normal cells, in the direct sample.



Case 2: CVS revealed low-level mosaicism for monosomy 21, showing loss of the maternal allele in a subset of cells. Amniocentesis was performed to resolve the discrepant NIPT and CVS results, which revealed high-level mosaicism for T21. The post-termination analysis provided additional insight by revealing non-mosaic T21 in the umbilical cord. Additionally, three placental biopsies exhibited varying degrees of mosaicism for T21, each showing a diallelic, trisomic pattern with double dosage of a single maternal allele. These results may be attributed to a mitotic nondisjunction event giving rise to three distinct placental cell lines and non-mosaic fetal abnormality.

Conclusion
Both cases feature non-mosaic abnormalities in the fetus, likely due to postzygotic errors occurring early in development, and a mix of normal and abnormal cell lines in the placenta. This contrasts with more common scenarios where a rescue mechanism restores diploidy in the lineage that gives rise to the fetus, resulting in the confinement of abnormal cells to the placenta and a normal fetal outcome. These cases highlight the complexity of prenatal diagnosis and the challenges of interpreting discrepant results in the presence of mosaicism.