Genetic counselling for trisomy 20 mosaicism in amniotic fluid cell cultures.

Trisomy 20 mosaicism in amniotic fluid cell cultures has an obscure origin and significance. In only a few cases has the mosaicism been confirmed in fetal tissues, but never in neonatal tissues. There is some indication that the trisomic cells might originate from epithelial cells deriving from the urinary tract. A review of 20 demonstrated cases reveals that no consistent malformation pattern is associated with this finding. Since it is questionable whether the presence of this mosaicism in amniocentesis can be regarded as a manifestation of abnormal fetal development, we suggest that it does not justify interruption of pregnancy. Postnatal cytogenetic studies and follow-up are indicated in these infants.