The use of Ultrasound in Diagnosis of Chromosomal Disorders

Most of us look forward to ultrasounds during pregnancy: it's a chance to get a look at our babies, hopefully get a picture or a video, and maybe even find out if it's a boy or a girl. But we usually overlook the fact that the real reason for ultrasound (also called a sonogram) is for the doctor to make sure that everything is progressing properly in the pregnancy. And for most pregnancies, that's what they find.

Learn more about Down Syndrome

But for some of us, that joyful, carefree event is changed when something unusual is noticed. Perhaps it's a choroid plexus cyst (as in our case) or the baby is small for gestational age. Or perhaps a triple screen result indicated higher than normal odds for a chromosomal condition. In those cases, a level 2 ultrasound is often recommended.

Also called a targeted ultrasound, this ultrasound is conducted specifically to look for particular markers based upon what was found in the initial ultrasound or triple screen. Before ours, we met with a genetic counselor, who explained what they were looking for and what that meant.

Level II ultrasounds are typically conducted by a perinatologist, fetal medicine specialist, or someone else who specializes in high-risk pregnancies and who is trained to look for the markers of chromosomal disorders. These markers are soft markers and structural markers.

What is Trisomy 13(Patau's Syndrome)?

Soft markers are characteristics frequently seen on babies with the disorder, and structural markers are abnormalities in organ and bone formation. Level 2 ultrasounds look for both of them. Genetic ultrasound explains why level II ultrasounds are much more accurate than standard screening ultrasounds.

And for many of us, it is at a level II ultrasound that we first hear the words "Trisomy 18" or "Trisomy 13" or hear "Down Syndrome" mentioned in the same sentence with our baby. And at that moment, our world changes.

Even if multiple markers are found in an ultrasound, this is still not a diagnosis

Although the term "level 2 ultrasound" is frequently used, the American Institute of Ultrasound in Medicine doesn't endorse the use of that term. And such terms as "level 3 or level 4 ultrasounds" are virtually meaningless. Interestingly, although the recently-introduced 3-D ultrasound is not used much in diagnosis yet, there is increasing evidence that it may someday be useful in identifying some of the more subtle soft markers of chromosome problems, such as low-set ears and cleft lip.

Some of the markers that may be seen via ultrasound for the three trisomies are:

Studies have shown that a single marker found on ultrasound is usually not a good indicator of a chromosomal condition (ref 5,6). In other words, if only one marker is found during a level 2 ultrasound, the odds are very high that the baby is healthy.

How Trisomy 18, Trisomy 13, & Down Syndrome are actually diagnosed

But even if multiple markers are found in an ultrasound, this is still not a diagnosis of a trisomy. To understand what it takes to receive an actual diagnosis of Edwards, Down, or Patau's Syndrome (Trisomy 13), see Edwards Syndrome, Down Syndrome, and Patau's Syndrome Resources.

For more information about ultrasounds and markers see Obstetric Ultrasound - A Comprehensive Guide and Ultrasound markers.

I am not a medical professional and the information included here is no substitute for medical advice; please consult your doctor.

Additional reading:

  1. Early second-trimester individualized estimation of trisomy 18 risk by ultrasound
  2. Ultrasonographic diagnosis of trisomy 18: is it practical in the early second trimester?
  3. Prenatal Screening for Down Syndrome
  4. Ultrasound Measurements for Down Syndrome
  5. Risk For Abnormal Chromosomes Following A Normal Genetic Ultrasound Examination
  6. Ultrasound's Down Syndrome Markers Often Unreliable
  7. Obstetric Ultrasound - A Comprehensive Guide

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This page updated September 10, 2004.