NUCHAL TRANSLUCENCY SCREENING / COMBINED SCREEN FOR FETAL ANEUPLOIDY

The First Trimester Combined Screen for Fetal Aneuploidy  is a natural evolution of screening tests for Down syndrome.  Advantages compared to second trimester biochemical screening  include:

  • Earlier diagnosis (11-14 weeks)
  • Higher detection rates  for fetal Down syndrome (80-90% or perhaps even higher compared to 75% for the second trimester "quad" screen and 60-70% for the older "triple" screen)
  • Detection of most major chromosome abnormalities other than trisomy 21
  • Acts as a nonspecific marker for other birth defects  including some major cardiac defects and syndromic conditions
  • Can detect a number of major structural birth defects associated with normal chromosomes

The primary disadvantage is

  • Narrow window of entry (11-14 weeks with ideal entry at 11-12 weeks).

This test integrates the 3 risk components that have been found to be effective in detection of fetal Down syndrome
during the first trimester:

  • Maternal Age (click here to see risk based on age alone)
  • Ultrasound measurements- nuchal translucency and crown rump length
  • Maternal biochemistry (free ß-hCG and PAPP-A) which can be obtained by a finger stick or blood draw

METHOD

The combined screen for fetal aneuploidy requires 3 components, as outlined by the Fetal Medicine Foundation.   

  • Appropriate counselling
  • Measurement of nuchal translucency by appropriately trained sonographers (FMF "certified" or equivalent)
  • Measurement of maternal serum free ß-hCG and PAPP-A by laboratories that can demonstrate good quality assurance performance.  The laboratories also typically have software that permit integration of ultrasound data and
    maternal age to provide a single overall risk estimate.   

 

RESULTS AND INTERPRETATION OF RESULTS

Results should return within a week or sooner.  The results will be faxed directly to the referring physician office. If the patient has had a formal counseling session, the genetic counselor or physician may phone the patient directly, if previously approved by the referring physician and patient.

Like 2nd trimester screening, results are typically returned as "Within normative range" or "positive".  The basis for these designations depends on the overall calculated risk, based on the combination of maternal age, ultrasound measurements, and maternal biochemistry.  The risk of fetal Down syndrome in a 35 year old woman (1:270) is typically used as the threshold for making this determination, so that all risks less than this will be termed "within normative range" and all risks higher than this are termed "positive".  Note that the positive indication does NOT indicate that the fetus has Down syndrome, only that the risk is considered to be high enough that the woman may want to consider further diagnostic testing.  It is far more likely that the fetus is completely normal. For example, a risk of 1:200 would be considered "positive" but the odds are that  199 of 200 cases the fetus will be normal.
 

Statement by the Fetal Medicine Foundation.  The Fetal Medicine Foundation promotes screening for Down syndrome at 11-14 weeks by Nuchal Translucency or a combination of nuchal translucency and maternal serum biochemistry.

Extensive research has now established that screening by nuchal translucency can detect about 80% of affected fetuses for a screen positive rate of 5%. The combination of nuchal translucency and maternal serum free ß-hCG and PAPP-A improves the detection to 90%