Later Prenatal Screening (first visit to health care provider is after 14 weeks of pregnancy)
If you have decided to have a prenatal screening test in the later stage of your pregnancy (after 14 weeks), you have the following options:
- You can have Triple or Quadruple prenatal screening. In most areas triple screening has been replaced by quadruple screening because it is a little more accurate.
- The steps involved in each of these tests and their accuracy are described in the table below.
Test |
Triple Screening |
Quadruple Screening |
Blood sample |
15 - 20 weeks |
15 - 20 weeks |
Results available at: |
16 - 21 weeks |
16 - 21 weeks |
Detection rate (Accuracy) |
Of every 100 pregnancies with Down syndrome, about 70 will be detected (70%) |
Of every 100 pregnancies with Down syndrome, about 75 to 85 will be detected (75-85%) |
False positive rate |
About 7 out of 100 pregnancies (7%) |
About 5 to 10 out of 100 pregnancies (5-10%) |
Diagnostic test if prenatal screening test is positive |
Amniocentesis
15 - 22 weeks |
Amniocentesis
15 - 22 weeks |
Diagnostic test results available at: |
17 - 24 weeks |
17 - 24 weeks |
Abortion – if you decide to have this – could be performed at: |
17 - 23 weeks
Abortion timing will depend on local availability |
17 - 23 weeks
Abortion timing will depend on local availability |
Or: Continuation with pregnancy |
Birth |
Birth |
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.
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