The Principles of Clinical Cytogenetics - Extra Materials - Springer

324 Solveig Pflueger
Table 1
Intrauterine Mortality per 100 Ova Exposed to Fertilization
Week after
ovulation Embryonic demise Survivors
— 16 (not fertilized) 100
0 15 84
1 27 69
2 5.0 42
6 2.9 37
10 1.7 34.1
14 0.5 32.4
18 0.3 31.9
22 0.1 31.6
26 0.1 31.5
30 0.1 31.4
34 0.1 31.3
38 0.2 31.32
Live births: 31
Natural wastage: 69
Source: Ref. 12.
(11), a level that is quite low compared with most other mammalian species. Leridon (12) provides a
useful summary table of pregnancy survival from fertilization to term, with only 31 survivors among
100 ova exposed to fertilization (see Table 1). Although most of the losses occur very early in gestation,
losses continue to occur throughout the second and third trimesters of pregnancy, with a slight
increase in mortality at term.
RELATIONSHIP BETWEEN CYTOGENETIC
ABNORMALITIES AND GESTATIONAL AGE
Multiple studies have suggested that approximately 50% of early pregnancy losses are associated
with cytogenetic abnormalities. Evaluation of 1205 pregnancy losses of varying gestational ages submitted
to the author’s laboratory between 1992 and 1996 revealed 539 (45%) cases with identified
cytogenetic abnormalities (13). The likelihood of a cytogenetic abnormality varies with the gestational
age and morphology of the abortus. In evaluating products of conception, the developmental age at
which growth arrest occurred is a more useful parameter than gestational age at the time of miscarriage,
because products of conception are often retained in utero for several weeks following embryonic demise.
Overall, the earlier the developmental age, the greater the likelihood of an abnormal karyotype in
a spontaneous pregnancy loss. Boué and colleagues (14) found that approximately two-thirds of losses
under 8 weeks and nearly one-fourth of those between 8 and 12 weeks had abnormal karyotypes (see
Table 2).
It is also of interest to note that the earlier the pregnancy undergoes growth arrest, the more likely it is
for there to be anomalous development and and that there will be an abnormal karyotype (see Table 3).
Gestational Age
Examination of induced abortuses confirms the greater incidence of karyotypic abnormalities earlier
in pregnancy (15) (see Table 4). A total of 1197 pregnancies were examined. The rate of chromosomal
abnormality varied with gestational age; 9.3% of cases were abnormal at 3–4 weeks, falling to
5.4% at 9–10 weeks.