The Principles of Clinical Cytogenetics - Extra Materials - Springer

Cytogenetics of Spontaneous Abortion 327
Table 6
Percent of Chromosomal Anomalies Among Spontaneous Abortions and Live Births
Anomaly
Autosomal trisomies
Spontaneous abortions Live births
13 1.10% 0.01%
16 5.58% 0.00%
18 0.84% 0.02%
21 2.00% 0.11%
Other 11.81% 0.00%
Total trisomies 21.33% 1.34%
Monosomy X 8.35% 0.01%
Sex chromosome trisomies 0.33% 0.15%
Triploids 5.79% 0.00%
Tetraploids 2.39% 0.00%
Total abnormal 41.52% 0.60%
Number karyotyped 3,353 31,521
Source: Adapted from ref. 19.
Cytogenetic abnormalities are a significant factor in human pregnancy wastage at all stages of
gestation, as well as into the neonatal period. However, the incidence of karyotypic abnormalities is
greatest during early pregnancy, with the majority of aberrant gestations resulting in early spontaneous
loss. Very early pregnancy loss is most likely to be the result of chromosomal abnormalities,
especially when there is evidence of marked embryonic growth arrest at the time of delivery. The
clinical significance of the loss and the potential impact on future reproductive risks for the couple is
dependent on the type of chromosomal error.
TYPES OF ERROR LEADING TO CHROMOSOMALLY
ABNORMAL CONCEPTUSES
Although most chromosomal abnormalities are associated with poor outcome early in pregnancy,
the underlying mechanisms leading to an aberrant karyotype and the risk for recurrence vary considerably
depending on the particular abnormal chromosomal complement. Generally speaking, most
karyotypic abnormalities fall into one of four classes: errors in meiosis (gametogenesis), errors in
mitosis leading to mosaicism, errors in fertilization, and structural abnormalities and rearrangements.
A classic study of 1498 abortuses by Boué and colleagues (20,21) revealed 921 abnormal karyotypes
(61.5%). Among the chromosomally abnormal losses were 636 nondisjunctional events: 141
monosomies (15.3%), 479 trisomies (52.0%), and 16 double trisomies (1.7%). There were 183 triploids
(19.9%), 57 tetraploids (6.2%) and 10 cases of mosaicism (1.1%). Structural abnormalities
were identified in 35 abortuses (3.8%). With improved cytogenetic and molecular methods being
used today, the incidence of detectable abnormalities might have been even higher. However, the
study clearly shows that cytogenetic abnormalities are present in the majority of early spontaneous
losses, and the data provide a useful breakdown of the types of abnormalities that are observed.
Normal karyotypes were seen in 577 abortuses (38%), although there may have been a few undetected
underlying abnormalities such as subtle rearrangements, uniparental disomy, or tissue-specific
mosaicism that could have gone undetected in this sample.
Analysis of 1205 products of conception of varying gestational ages received in our laboratory
between 1992 and 1996 revealed 539 (47.2%) abnormal karyotypes. Of these, 50.6% were trisomies,
11.3% were monosomies, 4.2% were tetraploid, and 14.8% were triploid (13). Although the total