November 2023 – Laboratory Genetics and Genomics

A 25-year-old woman with a twin gestation complicated by twin-to-twin transfusion syndrome underwent an amniocentesis procedure, and amniotic fluid from Twin A was obtained. There were no other anomalies or complications noted with the pregnancy. A chromosomal microarray study performed on cultured cells from the amniotic fluid showed mosaic trisomy 6 as displayed on the figure below. Maternal cell contamination was ruled out. 

Figure 1

Given this chromosomal microarray result, what is the most appropriate interpretation of this finding, and what subsequent action (if any) should be taken?

  • The microarray result is consistent with mosaic trisomy 6 in the fetus, which is incompatible with life, and no further action is needed.
  • The microarray result most likely represents a sample mix-up, as mosaic trisomy 6 is incompatible with life. A new amniotic fluid sample should be requested.
  • The microarray result represents pseudo-mosaicism due to culture artifact. As mosaic trisomy 6 is incompatible with life, no further action is needed.
  • The microarray result may represent pseudo-mosaicism due to culture artifact. Additional cytogenetic studies should be performed on an independent culture to confirm or refute this finding.

The correct answer is ...

The microarray result may represent pseudo-mosaicism due to culture artifact. Additional cytogenetic studies should be performed on an independent culture to confirm or refute this finding.

Mosaicism is defined as the presence of two or more populations of cells with differing genetic complements. When a mosaic finding is observed in prenatal specimens, several possibilities are important to consider when assessing if this represents true fetal mosaicism. In prenatal results from cultured amniotic fluid cells, apparent mosaicism may represent true fetal mosaicism, maternal cell contamination, or pseudomosaicism due to culture artifact. Since decisions regarding termination or continuation of a pregnancy can be influenced by these results, it is imperative to determine whether true fetal mosaicism is present or not. 

In this case, since maternal cell contamination has been ruled out, and low-level mosaicism could be present in a fetus with no obvious abnormalities by ultrasound, additional studies are required (karyotype, fluorescence in situ hybridization, or repeat microarray studies) to determine if true fetal mosaicism is present. These studies can either be performed on residual uncultured amniotic fluid specimen (if available) or on independent cultures derived from the same amniotic fluid sample that was received. If true fetal mosaicism is present, trisomy 6 cells should be detected in two or more independent cultures. If pseudomosaicism due to culture artifact is the cause of the mosaic trisomy 6, then the abnormality will only be present in the culture that was used for the chromosomal microarray study.


  1. Wegner RD, Entezami M, Knoll U, Horn D, Sohl S, Becker R. Prenatal diagnosis of fetal trisomy 6 mosaicism and phenotype of the affected newborn. Am J Med Genet A. 2004 Jan 1;124A(1):85-8. doi:10.1002/ajmg.a.20407. PMID: 14679592.
  2. Destree A, Fourneau C, Dugauquier C, Rombout S, Sartenaer D, Gillerot Y. Prenatal diagnosis of trisomy 6 mosaicism. Prenat Diagn. 2005 May;25(5):354-7. doi:10.1002/pd.1149. PMID: 15906424.
  3. Cockwell AE, Baker SJ, Connarty M, Moore IE, Crolla JA. Mosaic trisomy 6 and maternal uniparental disomy 6 in a 23-week gestation fetus with atrioventricular septal defect. Am J Med Genet A. 2006 Mar 15;140(6):624-7. doi:10.1002/ajmg.a.31129. PMID: 16470696.
  4. McKinlay Gardner, R. J, and others, 'Chromosome Abnormalities Detected at Prenatal Diagnosis', Gardner and Sutherland's Chromosome Abnormalities and Genetic Counseling, 5 edn, Oxford Monographs on Medical Genetics (New York, 2018; online edn, Oxford Academic, 1 Feb. 2018),

Sharri Cyrus, M.B.B.S.

Fellow, Laboratory Genetics and Genomics
Mayo Clinic

Nicole Hoppman, Ph.D.

Consultant, Laboratory Genetics and Genomics
Mayo Clinic
Associate Professor of Laboratory Medicine and Pathology
Mayo Clinic College of Medicine and Science

MCL Education

This post was developed by our Education and Technical Publications Team.