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Trisomy 18 (Edwards syndrome)

Trisomy 18 occurs when an individual has three copies of chromosome 18 instead of the customary two copies.  It happens due to a chromosomal imbalance in the sperm or more commonly, the egg that creates the baby.  The general population incidence of trisomy 18 is about 1 in 6000, but varies with maternal age.  Approximately 95% of pregnancies affected with trisomy 18 will result in miscarriage or stillbirth.  Of the liveborn infants, ~95% will die by 1 year of age.  Surviving infants will have severe intellectual disability and multiple congenital anomalies.
Prenatal ultrasound findings: congenital heart defects (90%), choroid plexus cysts, distinct hand posture, club and rocker bottom feet, micrognathia, microcephaly, intrauterine growth retardation and others.  Though rare, affected fetuses may have a normal ultrasound at 18-20 weeks.
Reference:
SOGC Clinical Practice Guideline: Fetal Soft Markers in Obstetric Ultrasound. J Obstet Gynaecol Can 2005; 27(6): 592-612.
Sanders, R.C., Blackmon, L.R., Hogge, W.A., Spevak, P., Wulfsberg, E.A. 2002. Structural Fetal Abnormalities the Total Picture. Mosby Inc.