The Principles of Clinical Cytogenetics - Extra Materials - Springer

332 Solveig Pflueger
Fig. 2. 45,X spontaneous loss at mid-gestation. Note marked cystic hygroma and generalized edema.
Whereas the mean maternal age for most trisomic conceptuses is increased over the normal population,
this is not the case with monosomy X. Rather, the mean maternal age for monosomy X is the
same or lower than expected for the reproductive age population as a whole (14). The evidence that
many cases of monosomy X are the result of postzygotic nondisjunction might possibly explain the
difference in maternal age between aneuploid pregnancies with monosomy X and those with autosomal
trisomies. Mitotic nondisjunction during embryogenesis appears to be a different process, which
might not exhibit the same maternal age effect, hence the maternal age for monosomy X would not be
expected to be increased over the mean reproductive age of the population. Although patients who
have experienced a pregnancy with monosomy X often choose to have prenatal cytogenetic evaluation
in subsequent gestations, the recurrence risk for postzygotic/mitotic nondisjunctional events has
not been established.
Errors in Mitosis
Malsegregation in the first mitotic division can give rise to tetraploidy. Tetraploid conceptions are
usually lost relatively early in gestation, although there are rare exceptions.
Mitotic nondisjunction often results in mosaicism—the presence of two or more cell lines with a
different genetic makeup. As has been suggested for Turner syndrome, mosaic aneuploidy might be
better tolerated by the developing conceptus than complete aneuploidy, and there is evidence that
survival of a trisomic fetus to term might be more likely if there is a normal cell line present within
the placenta.
The question of tissue specific mosaicism has long been an issue in prenatal diagnosis, especially
with the advent of chorionic villus sampling (CVS) (see Chapter 12). Early nondisjunction can result
in a generalized pattern of mosaicism, whereas divergence later in gestation can lead to mosaicism
confined to either the fetus or the placenta. Mosaicism confined to the amnion could present a dilemma
in interpretation of amniotic fluid karyotype, yet might not pose a problem for the fetus (41). Within