Prenatal diagnosis of trisomy 21 by i(21q): a rare case of fetoplacental chromosomal discrepancy

OBJECTIVE: A study was conducted to explain the mechanism of an unusual discrepancy between short- and long-term culture examination methods of chorionic villus sampling (CVS). METHOD: In a 29-year-old Caucasian woman, transabdominal CVS was carried out at 12 weeks of gestation. Non-mosaic karyotype 46,XX,i(21q) was found on long-term CVS culture but number and morphology of chromosomes were normal on short-term culture, amniocyte culture, hygroma colli fluid and fetal fibroblast. RESULTS: Chromosomal aberration probably appeared after the trophoblast cell line differentiation, four days after fertilization, by means of a 21 centromere misdivision and formation of a i(21q) with secondary positive selection of the 46,XX,i(21q) cell line and loss of the 46,XX in the fetus. CONCLUSION: The restricted number of cases with this type of discrepancy limits the possibility of drawing generalised conclusions. In case of discrepancy, we recommend confirmation by amniocentesis or by fetal blood combined with sonographic examination to provide a more definitive diagnosis.