The First Trimester Risk Assessment

Introducing The First Trimester Risk Assessment

In 2005, The New England Journal of Medicine published data that definitively proved that the First Trimester Risk Assessment (or First Screen) is the most accurate non-invasive screening test to determine whether or not a mother is at risk of having a child with a chromosomal abnormality-namely Down’s Syndrome (Trisomy 21) or Trisomy 18.The First Screen does not screen for neural tube defects such as Spina Bifida or anencephaly.

Before the First Screen, physicians and couples had to rely on the far less dependable screening tests for Alpha FetoproteinThe Triple Screen or The Quad Screen to screen for risk of chromosomal abnormalities and neural tube defects in the fetus. The high false positive rates of these tests and the subsequent invasive and riskier diagnostic tests (amniocentesis and chorionic villus sampling) that often followed make The First Screen a welcome addition to prenatal screening tests. With an 85% accuracy rate and a low 5% false positive rate, couples that come back with a negative test can be 85%  sure that their baby is developing just fine. Likewise 5% of all couples tested will have a positive test and yet will go on to have a healthy baby without defect.

How The First Screen is Done

The First Screen is done between 11 and 13 weeks of pregnancy. Mother first has blood drawn to measure levels of pregnancy-associated plasma protein-A (PAPP-A) and Human Chorionic Gonadotropin (hcG). Then a high resolution ultrasound is performed to measure the nuchal translucency (fluid beneath the skin on the back of the neck) of the fetus. The results of the blood tests along with the results from the ultrasound and the mother’s health information combined to give a risk assessment for  genetic abnormality for the fetus.

What do the results mean?

If the screening test comes back “normal” then there is little risk of the fetus developing a chromosomal abnormality and is at low risk for Down’s Syndrome or Trisomy 18. If the screening test comes back abnormal, then a diagnostic test may be recommended to determine if a true genetic abnormality exists. Mother and her partner will be counseled by genetic counselors in addition to their obstetrician and/or perinatologist what to do next.