Autosomal mosaicism in amniotic fluid cells from a twin pregnancy

Amniocentesis was performed on a 39-year-old gravida 4 woman because of maternal age. Ultrasonography demonstrated a twin pregnancy, and two amniotic fluid specimens were obtained under direct ultrasound guidance. All cells analyzed from the first specimen were 46,XX. In the second specimen, one colony in each of two flasks had trisomy 9; the remaining 3 colonies analyzed were 46,XX. The total cell count in the second specimen was 14 (24%) of 58 cells which were trisomic for chromosome 9. The parents elected to continue the pregnancy, which resulted in two live-born infants, a male and a female, both chromosomally normal. The most likely explanation for the amniotic fluid finding is that fluid from one sac was sampled twice and that trisomic cells from the amnion were obtained during the second tap.