What is the role of ultrasonography in prenatal screening of Down syndrome?

Updated: Apr 30, 2018
  • Author: Gratias Tom Mundakel, MBBS, DCH; Chief Editor: Maria Descartes, MD  more...
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Answer

 Prenatal ultrasonography may reveal the following in a fetus with Down syndrome:

  • Ultrasonography soft markers for Down syndrome observed in the second trimester include absent or hypoplastic nasal bone, thickened nuchal fold, echogenic bowel, shortened long bones, and pyelectasis

  • Absent or hypoplastic nasal bone is observed in 43-62% of trisomy 21 fetuses, compared with 0.5-1.2% of normal fetuses

  • A thickened nuchal fold has been associated with a greatly increased risk of trisomy 21 and may be an early feature of fetal hydrops or cystic hygroma; measurement of NT in the first trimester, as discussed above, can detect up to 70% of fetuses with Down syndrome

  • Echogenic bowel has been observed in approximately 15% of fetuses with trisomy 21, compared with 0.6% of normal fetuses; about 35% of fetuses with true echogenic bowel have some underlying pathology, such as first trimester bleeding, fetal infections, and cystic fibrosis due to meconium ileus

  • Shortened long bones (humerus and femur) have been associated with an increased risk of chromosomal abnormalities; the humerus is a more reliable discriminator for Down syndrome than the femur and appears to be the next most important marker after nasal bone and nuchal fold

  • Pyelectasis has been observed in approximately 17% of fetuses with trisomy 21, and approximately 1 in every 300 fetuses with isolated pyelectasis has aneuploidy; pyelectasis has been associated with an increased risk of hydronephrosis and postnatal urinary reflux

  • Other ultrasonographic abnormalities include cystic hygroma, duodenal atresia or stenosis (double-bubble sign), cardiac defects (endocardial cushion defect with atrial and ventricular septal defects and abnormal mitral and tricuspid valves), intracardiac echogenic focus, and prune belly anomaly

Ultrasonography should not be relied on as the primary method of diagnosing Down syndrome; the diagnosis can be missed in affected families. Suggestive prenatal ultrasonographic findings may be followed with amniocentesis and fetal chromosome analysis.


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