+ 0 ) . trisomy 20 mosaicism ( 45 , X / 46 , X , syndrome in conjunction with Prenatal detection of Turner ' s

SUMMARY A case of Turner's syndrome, detected antenatally and complicated by the finding of trisomy 20 mosaicism in 50 % of cells from each of two amniotic fluid cultures, is described. Cultures from seven fetal tissues in the subsequent abortus showed a predominance of 45,X cells, but nevertheless suggested the existence of a very low level of trisomy 20 mosaicism in three fetal tissues. The diagnostic dilemma in interpreting trisomy 20 mosaicism is discussed. Much discussion has centred round the definition of mosaicism in cultured amniotic fluid cells. It is considered to be likely to exist when an identical abnormal cell line is found in more than one culture or clone. However, in some cases mosaicism has not been verified in the resultant abortus.1 2 Simola et al3 suggest that mosaicism should be ignored except where there are known severe consequences. Trisomy 20 may be in the latter category since an association with severe mental retardation has been reported.4 We report an example with monosomy X and mos-aicism for trisomy 20. Counselling problems are discussed and essential procedures for confirmation are outlined. Case report A 40-year-old woman in her third pregnancy was referred for amniocentesis because of her age. She had had a normal delivery 7 years previously, followed by a spontaneous first trimester abortion. Amniocentesis was performed at 18 weeks. Materials and methods Clear amniotic fluid was obtained. Flask cultures were initiated using Ham's FIO supplemented by 30% fetal calf serum. Seven different tissues from the subsequent abortus (table) were taken to confirm the diagnosis. Both flask and coverslip cultures were established from each tissue using similar growth medium. Sixty cells were analysed from each tissue to exclude 5 % mosaicism to the 95 % confidence limit.5 Results Liquor ox-fetoprotein concentration was normal (14 ,ug/ml) while cytogenetic analysis revealed no normal karyotypes. The table shows the chromosome results for all amniotic fluid and fetal tissue cultures. M\anagement The missing sex chromosome from all cells made the interpretation of the additional F-like chromosome difficult since it could have been derived from a sex chromosome, for example, iso Yq or Xp-q-. However, the G banding pattern strongly suggested identification of the F as a No 20 (fig 1). Counselling was difficult because we could find no precedent in published reports. The parents were advised that the fetus would be likely to have the Turner phenotype but that this could be modified …