Trisomy 20 mosaicism detected prenatally: A case report of phenotypically normal male liveborn

Mosaic trisomy through chorionic villus sampling is extraordinarily rare, but it is one of more common mosaic trisomies detected on amniocentesis. Its clinical significance remains uncertain due to rarity of live born cases with inconsistent clinical findings and short postnatal follow-up. There is a clear association between the percentage of trisomic cells detected on amniocentesis and outcome, with only 5% abnormal outcomes when <50% trisomic cells were detected. Trisomic cells are practically never confirmed in newborn blood with increasing risk for abnormal outcome in case of positive confirmation and only rarely are found in other fetal tissues samples.

We report a case of the phenotypicaly normal live born male after prenatal diagnosis of trisomy 20 mosaicism in amniocytes. This is second child in order, first child is healthy. Mother is 39 years old. Cytogenetic analysis of the amniotic fluid sample showed that mosaic tissues are consisted of two cells types, which are different in number of chromosomes: normal cells with 46 chromosomes and trisomic cells with 47 chromosomes. In trisomic cells there was a one more chromosome No 20. 31 cells of amniotic fluid were analyzed: 25 normal mitoses (46XY) and 6 trisomic mitoses (47XY) were detected. Parents were counseled that the risk of abnormalities is less than 10% and they decided to keep the pregnancy. A phenotypically normal male baby was born. He passed neonatal period, and for now, he is developing normally.

Cytogenetic analysis was performed postnatally on lymphocytes and trisomy 20 cells could not be confirmed after birth. The origin of trisomy 20 cells in amniotic fluid remained unclear.

Prenatally diagnosed trisomy 20 mosaicism required further evaluation and parental counseling. Although risk for congenital abnormalities in low level of trisomic cells is less than 10%, parents should be advised of an additional risk, such as developmental delay.

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Trisomy 20 mosaicism - amniocentesis - outcome