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Trisomy 18 (Edwards Syndrome)
Trisomy 18 is a chromosomal anomaly in which the fetus has an extra chromosome 18 (i.e., it has three, instead of the normal two, chromosomes 18). It is a rare anomaly, occurring in approximately three per 10000 births. Like other trisomies, the incidence increases with advancing maternal age. Fetuses with trisomy 18 generally have severe structural anomalies involving multiple organ systems. Most die within the first year of life, and the few long-term survivors are severely neurologically impaired.

Many of the structural anomalies in fetuses with trisomy 18 can be identified by ultrasound. Among these are the following:
     Central nervous system
          Agenesis of the corpus callosum
          Choroid plexus cysts
          Hypoplastic cerebellum with enlarged cisterna magna
          Clenched hand with overlapping index finger
          Rockerbottom foot
     Diaphragmatic hernia
     Cardiac anomalies
     Renal anomalies
     Intrauterine growth restriction

When a constellation of anomalies, including several of the above, is detected by ultrasound, the diagnosis of trisomy 18 should be considered and amniocentesis should be offered. Furthermore, several anomalies that occur in trisomy 18 are associated with an increased incidence of aneuploidy even when seen as an isolated finding (e.g., microphthalmia, clenched hand with overlapping index finger, rockerbottom foot, omphalocele, diaphragmatic hernia) and thus are indications for amniocentesis.
Fetuses with trisomy 18 have an increased incidence of choroid plexus cysts in the second trimester. There is some disagreement about whether amniocentesis should be offered when ultrasound demonstrates choroid plexus cysts and no other structural anomalies. It is generally agreed, however, that amniocentesis should be offered when the fetus is found to have choroid plexus cysts together with another anomaly, especially one of the anomalies listed above.

Trisomy 18 with a cystic hygroma: Axial view of the head demonstrates a cystic hygroma (arrow) in the occipital and posterior nuchal areas.

Trisomy 18 with ascites: Transverse view of the abdomen demonstrates ascites (arrow).

Trisomy 18 with hypoplastic left ventricle: Four-chamber view of the heart demonstrates a hypoplastic left ventricle (long arrow) with a normal right ventricle (short arrow).

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