Mosaic autosomal trisomy in cultures from spontaneous abortions

In a consecutive series of 592 karyotyped spontaneous abortions, ten of 103 autosomal trisomies were mosaic, with a normal cell line also present. The frequency of mosaicism (10%) is much higher than that reported in Down syndrome, but similar to that reported in amniotic fluid cultures and in induced abortions. The most likely explanations for this discrepancy are (1) previous underestimation of mosaicism in live births or (2) mosaicism which is often restricted to extraembryonic fetal tissue.