Small marker chromosomes in a series of 1,000 prenatal diagnoses by amniocentesis.

We diagnosed two small marker chromosomes in a series of 1,000 prenatal cytogenetic studies of amniotic fluid cells. Each of these chromosomes was analyzed with various staining techniques in order to determine its structure and the possible mechanism of its formation. On the basis of the results thus obtained and the familial nature of these abnormalities, we predicted phenotypically normal fetuses in both cases. Postnatal follow-up confirmed this. Notwithstanding the correct diagnoses made in these two cases, we feel that a more substantial body of literature on this type of anomaly must become available before it will be possible to give firm genetic counselling in such cases.