ultrasound diagnosis of fatal anomalies

CHROMOSOMAL DISORDERS
Trisomy 21 (Down Syndrome)
Definition: Numerical chromosomal aberration,
with an extra chromosome 21. This is the most
common trisomy in live-born neonates. The
phenotype (Down syndrome) is characterized by
severe malformations and developmental disorder.
Cardiac anomalies and mental impairment
are the commonest features.
Incidence: One in 700–800 births.
Sex ratio: M:F=1:1.
Clinical history/genetics: The incidence of Down
syndrome correlates with maternal age. As in
trisomy 13, three forms of chromosomal aberrations
are found in trisomy 21: free trisomy, found
in about 95% of patients with Down syndrome;
translocation trisomy 21, affecting about 3%; and
trisomy 21 mosaic, affecting 2% of patients.
In free trisomy 21, the recurrence risk lies 1%
above that for the maternal age. Only 30% of
trisomy 21 cases would be diagnosed if all
mothers over the age of 35 had chromosomal
analysis carried out during prenatal screening.
Using the triple test, prenatal diagnosis can be
Fig. 9.54 Trisomy 21. Longitudinal section at
11+1weeks,showing hygroma colli.
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Fig. 9.55 Trisomy 21. Same fetus as before. A
cross-sectional view demonstrates the presence of
septa within the hygroma.
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