A Level 2 Ultrasound, also called a targeted ultrasound, is conducted specifically to look for particular markers based upon what was found in the initial ultrasound or triple screen. Perhaps the initial ultrasound found a choroid plexus cyst 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. This is another screening test that does not diagnose Trisomy 18 but gives more information about the odds of its presence.
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 typically conducts level 2 ultrasounds. These markers are soft markers and structural markers. Soft markers are characteristics frequently seen on babies with the disorder, and structural markers are abnormalities in organ and bone formation.
Some of these markers for Trisomy 18 are as follows:
clenched hands, choroid plexus cysts, rocker bottom feet, delayed growth, heart defects such as VSD, ASD, and coarctation, kidney abnormalities, omphalocele, esophageal atresia, and polyhydramnios (excess amniotic fluid)
(see What is Trisomy 18 for more information about these markers)
What if they only find one marker?
Studies have shown that most (but not all) babies with Trisomy 18 show some abnormalities during ultrasound (ref 1,2). Other studies have shown that a single marker found on ultrasound is usually not a good indicator of a chromosomal condition (ref 3,4). In other words, if only one marker is found during a level 2 ultrasound, the odds are very high that the baby is healthy.
What if they find several markers?
But even if multiple markers are found in an ultrasound, this is still not a diagnosis of a trisomy. Remember that screening tests cannot result in an absolute diagnosis. See How Trisomy 18 is diagnosed for an explanation of the difference between screening and diagnostic tests.
If several markers are found, typically the next step is an amnio, which is a diagnostic test. See Amniocentesis for more information about this test.
For more information about ultrasounds and markers see Obstetric Ultrasound - A Comprehensive Guide and Ultrasound markers
- Early second-trimester individualized estimation of trisomy 18 risk by ultrasound
- Ultrasonographic diagnosis of trisomy 18: is it practical in the early second trimester?
- Risk For Abnormal Chromosomes Following A Normal Genetic Ultrasound Examination
- Ultrasound's Down Syndrome Markers Often Unreliable