A 44 year-old patient, G₂ P₁,  with an unremarkable medical history. No medical history.Previous child 10 year-old, healthy. This is her 2nd pregnancy (with a different father)

First scan at 12 week, regarded as normal: nasal bone visible (not visible on this picture), NT 1,9/56 mm, Doppler DV positive "a" wave

An amniocentesis (for maternal age) was performed at 19 week.
The ultrasound findings included growth retardation (< 3rd percentile), ventricular septal defect, and negative ductus venosus "a" wave.

The cardiac anomaly ressembles a tetralogy of Fallot

Following this examination, an amniocentesis was performed which revealed an abnormal karyotype.

Cytogenetic results:

1st  cytogenetic result :

Chromosome mosaicism with a supernumerary chromosome looking like an isochromosome 21q.
In vitro chromosomal abnormality was excluded because four abnormal colonies were found in three different culture vessels.
From the cytogeneticst point of view, two dilemmas have to be resolved:

• discrimination of the isochromosome 21q (or mosaic tetrasomy 21q) from the isochromosome 12p (or mosaic tetrasomy 12q) because photographs of these chromosomes are quite similar and mosaic tetrasomy 21q is much rarer than mosaic tetrasomy 12p

• discrimination of "true chromosome mosaicism" and "pseudomosaicism" in which the cells with the abnormal karyotype derive from placental tissues.

In situ hybridisation (FISH) is an efficient technique for distinguishing isochromosome 21q from isochromosome 12p.
                  
Fetal blood sampling may help us to distinguish a "true chromosome mosaicism" from "pseudomosaicism".

2nd and final cytogentic result :

Mosaic tetrasomy 21q was confirmed by FISH and the fetal karyotype was written as:

47,XX,+i(21)(q10)/46XX

46, XX normal

tetrasomy 21q

A microdeletion in the DiGeorge chromosome region of chromosome 22 was searched because of the fetopathology findings (ie: thymus aplasia) and was excluded by FISH.

- FISH  21: 4 red spots

- FISH  22: red : locus q11.2; green : control

As the FISH technique was underway, and discussion was taking place about the opportunity of fetal blood sampling, the patient was scanned again at 21 week for control. The examination demonstrated intra uterine death with severe IUGR below 3^rd percentile and fetal hydrops.

Fetopathology findings: 21 trisomy dysmorphology, pulmonary atresia with ventricular septal defect (PA-VSD, the old "extreme form of Fallot"), thymus aplasia.

References:

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9:  Hall BD. Mosaic tetrasomy 21 is mosaic tetrasomy 12p some of the time.Clin Genet. 1985 Mar;27(3):284-5. No abstract available. PMID: 3987079 [PubMed - indexed for MEDLINE]

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