Prenatal losses of trisomy 21.

Various cytogenetic studies of early spontaneous abortions have observed trisomy 21 in about 3% of the abortuses. Analyses of prenatal diagnosis performed during the 17th week of gestation and observations on late spontaneous abortions and perinatal deaths have also shown the importance of lethal trisomy 21 in the second half of pregnancy. From these studies, it can be estimated that at least four-fifths of conceptuses with trisomy 21 fail to survive. One important question is why the same chromosome anomaly in a zygote can lead either to an early arrest of development or to a liveborn infant. Pathologic examinations of spontaneously aborted embryos with trisomy 21 have rarely shown malformations that can explain the death of the embryo, but macroscopic and microscopic examinations of their placentas show growth retardation and hypoplasia. Analysis of in vitro growth characteristics of cell lines established from tissues of trisomy 21 embryos have demonstrated differences in cell population doubling times; this may explain variations in the developmental potentials of conceptuses with trisomy 21.