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Down Syndrome (Trisomy 21)

Down syndrome occurs when an individual has three copies of chromosome 21 (trisomy 21) 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 birth rate of Down syndrome is about 1 in 1000, but varies with maternal age.  Features of Down syndrome can include intellectual diability of varying severity, characteristic facial appearance, hypotonia and other congenital anomalies.
Prenatal ultrasound findings: soft markers (such as echogenic intracardiac focus, absent nasal bone, echogenic bowel, increased nuchal translucency measurement), congenital heart defects (40%), intestinal obstruction (12%), approximately 1/3 of affected fetuses will have normal ultrasounds 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.