Chromosome mosaicism in 6,000 amniocenteses.

Multiple cell-multiple flask mosaicism was found in 0.20% of 6,000 amniocenteses, and multiple cell-single flask mosaicism was found in 0.92%. Multiple cell-multiple flask mosaicism usually was found in fetal or infant tissues at delivery or elective abortion. Most multiple cell-multiple flask mosaicism involved sex chromosomes and was either 45, X/46, XY or 45, X/46, XX. Except for one fetus with 45, X/46, XX and an aortic coarctation, phenotypic abnormalities associated with sex chromosome mosaicism were not found in these patients. One normal boy has continued to show 45,X mosaicism during the first 4 years of life. Autosome abnormalities found in multiple cell-multiple flask mosaicism included del(18q) associated with fetal anomalies. Apparently normal phenotypes were associated with prenatal trisomy 17, two de novo supernumerary marker chromosomes, and monosomy 21. Since an aberrant cell line present in only one primary amniotic fluid cell culture was occasionally identified from another amniocentesis or at birth, multiple cell-single flask mosaicism involving a sex chromosome or a viable autosome abnormality cannot be assumed to be an in vitro event. Maternal cell contamination, which was found in 0.49% of amniocenteses, could have resulted in an erroneous diagnosis of fetal sex in two cases if cells from independent culture vessels were not examined.