How accurate are prenatal screening tests?
Down syndrome The detection and false positive rates in the table only refer to Down syndrome. This is because trisomy 18 is rare and we do not have exact numbers for it. Trisomy 18 Generally, for all of the tests listed, the detection rate or accuracy is less for trisomy 18 than what it is for Down syndrome. Said another way, these tests are not quite as good at measuring the chance of having a baby with trisomy 18 (detection rate) as measuring the chance of having a baby with Down syndrome. Open neural tube defects Screening for open neural tube defects consists of a blood test done between 15-20 weeks. This is done as part of IPS, SIPS, Quad or Triple screening or as an additional blood test if a woman is having FTS. The ultrasound done at about 18 weeks of pregnancy also gives information about open neural tube defects. Of every 100 pregnancies with an open neural tube defect, 80 (or 80%) will be detected with prenatal screening. Remember that most babies are born healthy.
