Ultrasonographic Soft Markers of Aneuploidy in Second Trimester: Are We Lost?

Sameer Raniga, MD, DNB; P.D. Desai, MD; Hetal Parikh, MD

Disclosures
In This Article

Introduction

Chromosomal abnormalities occur in 0.1% to 0.2% of live births.[1,2] Trisomy 21 (Down syndrome) is the most common karyotypic abnormality in live-born infants (1 per 800 live births)[3] and is a leading cause of mental retardation. Sonographic findings in fetuses with Down syndrome include both structural abnormalities and nonstructural abnormalities or "markers."[4,5,6] Other sonographically detectable aneuploidies include trisomy 13, trisomy 18, monosomy X, and triploidy.

Various methods have been used to identify women at risk of carrying a fetus with trisomy 21, including consideration of maternal age,[2] biochemical markers,[7] amniocentesis,[8,9] and prenatal ultrasound. Amniocentesis can reliably determine fetal karyotype, but there is a 0.5% to 1.0% fetal mortality rate associated with this procedure.[8,9]

A second-trimester ultrasound scan is usually done at 18 to 22 weeks. Two types of sonographic markers suggestive of aneuploidy can be observed in the second trimester. Major fetal structural abnormalities comprise the first type ( Table ). There are many other, less-defined features that have been given less significance as "possible markers" of aneuploidy, and these are collectively called "soft markers" of aneuploidy ( Table ). Although not pathologic themselves, these markers have been used to screen for, or adjust the risk for, Down syndrome and other aneuploidies.[10,11] Soft markers may be seen in the normal fetus but have an increased incidence in infants with chromosomal abnormalities. These markers are nonspecific, often transient, and can be readily detected during the second-trimester ultrasound.[12] Thus, prenatal ultrasonography during the second trimester provides a "genetic sonogram" that is used to identify morphologic features of fetal Down syndrome.[13]

For a number of years, members of the ultrasound community involved in obstetric sonography have been grappling with a controversial issue centered on soft markers of aneuploidy. Major abnormalities are observed in fewer than 25% of affected fetuses in most studies,[4,14,15,16] whereas 1 or more soft markers may be observed in at least 50% of cases.[14,17,18] Prenatal ultrasound attempts to detect the soft markers; ultrasound in the second trimester currently diagnoses 50% to 70% of cases of Down syndrome, 70% to 100% trisomy 18,[19,20] and 90% to100% trisomy 13.[1.]

The most commonly studied soft markers of aneuploidy include a thickened nuchal fold, rhizomelic limb shortening, mild fetal pyelectasis, echogenic bowel, and echogenic intracardiac focus (EIF) and choroid plexus cyst (CPC). There is a great deal of interest in the ultrasound detection of aneuploidy, as evidenced by the large number of publications in the literature on this topic. Unfortunately, studies evaluating the significance of the soft markers of aneuploidy vary widely and show contradictory results. We review the most common ultrasonographic soft markers used to screen aneuploidy and discuss ultrasonographic technique and measurement criteria for the detection of soft markers. We also review the clinical relevance of soft markers to aneuploidy risk assessment and evidence-based strategies for the management of affected pregnancies with each of these markers in light of current literature.

Some of the sonographic markers of aneuploidy are described in the Table .

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