Down Syndrome Screening

Combined First Trimester Screen

Combined first trimester screening is a screening test for Down Syndome/Trisomy 21 (detection rate is 90% and the false positive rate 5%).

• It combines placenta protein levels (blood tests for PAPPA and free hCG) with age related risk, nuchal translucency (NT) measurement (fluid behind the neck) and nasal bone appearance in a special computerised algorithm to determine the pregnancy risk for Down syndrome
• The blood test is best done around 10 weeks gestation. We recommend that it is done at least 2 weeks before the scan to reduce the turn around time for the final result
  
• The scan is best done between 12 and 13 weeks (although it is possible between 11 3/2 – 13 6/2 as long as the fetus must be 45-84mm in CRL length to be able to measure the NT))
• When the risk for Down Syndome is returned, it is a risk number. The cut off between high and low risk is 1:300. Anything more likely than 1:300 is in the high risk range, with the highest risk being 1:2.
• If a high risk result is returned then discussion with your doctor is recommended to determine whether an invasive diagnostic test or the more sensitive NIPT screening test  or an invasive diagnostic test may be the next appropriate step.