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Ultrasound Markers for Down Syndrome

Down Syndrome (trisomy 21) is the most common chromosomal disorder in live born infants. About 6,000 babies are born with Down Syndrome each year in the United States, or about 1 in every 700 births.  There are multiple prenatal genetic screening strategies and diagnostic tests aimed at accurate prenatal identification of Down Syndrome and other aneuploidies, with ultrasonographic evaluation being an essential diagnostic modality.

Both major structural abnormalities and minor “soft markers” can be detected by ultrasound in fetuses affected with aneuploidies. Major abnormalities may involve the fetal cardiovascular, central nervous, craniofacial, musculoskeletal, gastrointestinal, and urinary tract system abnormalities including duodenal atresia, atrioventricular canal defects, and tetralogy of Fallot.

Soft markers include thickened nuchal fold, ventriculomegaly, absent nasal bone, hyperechogenic bowel, choroid plexus cyst, short femur, echogenic intracardiac focus, mild tricuspid regurgitation, pyelectasis, and single umbilical artery. These soft markers are non-specific and do not completely indicate a structural abnormality of the fetus, potentially representing a normal variation.

In the first trimester, ultrasonographic evaluation involves assessing the subcutaneous fluid-filled space between the back of the fetal neck and the overlying skin termed “nuchal translucency” (NT).  There is an association between increased NT and risk of aneuploidies, with the detection rate for Down Syndrome being 64-70%. A nuchal cystic hygroma represents pathologic nuchal edema and is associated with aneuploidy in about 50% of cases. The nasal bone is considered absent when it is not visualized on a midsagittal view of the profile A hypoplastic or absent nasal bone can be detected in the first trimester in 62-70% of fetuses with Down Syndrome.

In the second trimester, the aforementioned soft markers may be identified. The presence of echogenic bowel, thickened nuchal fold, or ventriculomegaly, even when isolated, is prompts further aneuploidy screening or diagnostic testing.  An isolated finding, an increased nuchal skin fold confers the highest risk of aneuploidy and is the most powerful second trimester ultrasound marker, with > 99% specificity for Down Syndrome.

Accuracy in detecting Down Syndrome is increased when ultrasound findings are interpreted in combination with serum analyte screening tests such as first and second trimester screening and integrated and sequential screening, which has largely been replaced with cell-free DNA techniques which confer a detection rate of 99% for Down Syndrome.  Diagnostic testing, such as amniocentesis or chorionic villus sampling, is required to confirm a diagnosis of Down Syndrome when screening test results are positive.

Further Reading:

American College of Obstetricians and Gynecologists’ Committee on Practice Bulletins—Obstetrics; Committee on Genetics; Society for Maternal-Fetal Medicine. Screening for Fetal Chromosomal Abnormalities: ACOG Practice Bulletin, Number 226. Obstet Gynecol. 2020 Oct;136(4):e48-e69. doi: 10.1097/AOG.0000000000004084. PMID: 32804883.

Cai M, Lin N, Chen X, Fu M, Guo N, Xu L, Huang H. Evaluation of chromosomal abnormalities and copy number variations in fetuses with ultrasonic soft markers. BMC Med Genomics. 2021 Jan 12;14(1):19. doi: 10.1186/s12920-021-00870-w. PMID: 33435955; PMCID: PMC7802188.

Committee on Practice Bulletins—Obstetrics and the American Institute of Ultrasound in Medicine. Practice Bulletin No. 175: Ultrasound in Pregnancy. Obstet Gynecol. 2016 Dec;128(6):e241-e256.

Initial approval: November 2017. Reaffirmed May 2019, Reaffirmed January 2021, Minor Revision July 2022.  Revised September 2024.

 

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