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Non Invasive screening tests for Downs Syndrome, Trisomy 18 and Open Spina Bifida

First Trimester Integrated Screen
The first trimester integrated screen involves two separate blood tests, and a special ultrasound between 11-13 weeks gestation, to screen for Down's Syndrome (Trisomy 21), Trisomy 18, and also open spina bifida.  The first blood draw is done usually at the same time the ultrasound, between 11-13 weeks gestation.  The ultrasound measures the thickness of the baby's neck fold, called the nuchal translucency.  The second blood draw is done between 15 and 21 weeks gestation.
This test can lead to the detection of 92% of the babies with Down's Syndrome, 90% of babies with Trisomy 18, and 80% of babies with open spina bifida.  A positive test does not necessarily mean that the baby has any of these conditions.  Further testing would be offered for further evaluation. 
The false positive rate for this test (false positive means that the tests results are abnormal, but that the baby does not have the condition) is 5%.  If a women has a positive test indicating an increased risk, the odds of the baby being affected are 1 in 21.
Serum Integrated Screening
The Serum Integrated Screen has the same two blood tests as the First Trimester Integrated Screen test, but does not include the nuchal translucency measurement.  The detection rate for Down's Syndrome in this test, compared to the First Trimester Integrated Screen, drops 5 percentage points, from 92% to 87%.  The detection rates for Trisomy 18 and open spina bifida are the same, being 90% and 80% respectively. 
The false positive rates with the Serum Integrated Screen is 5%.  The odds of being affected for the Serum Integrated Screen (the odds that the baby will be affected given an abnormal test result are 1 in 22.
AFP Multiple Marker Test (Quad Test)
The multiple marker test involves testing the Maternal Alpha Serum Fetoprotein, as well as three other biochemical markers in the maternal blood.  This test is time dependent, and involves a single blood test between 15-21 weeks gestational age.  The Quad test can lead to the detection of 81% of babies with Down Syndrome, 80% of babies with Trisomy 18 and 80% of babies with open spina bifida.
The false positive rate is 5%.  The odds of having open spina bifida, given an abnormal Quad test, are 1 in 23.
Remember that these tests are screening tests only.  An abnormal result does not necessarily mean that the baby has an abnormality.  If your test is abnormal, you will be offered further evaluation and testing.  Such testing will involve a detailed ultrasound and possibly the option of an amniocentesis.  Amniocentesis involves the removal of a small amount of amniotic fluid from the uterus with a needle, and testing the fetal cells in the fluid.  The procedure is done under ultrasound guidance.  There is a small risk of miscarriage of 1 in 200 to 1 in 400 in women having the procedure.

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