1. First trimester Screening for Aneupoidy:
- Double Marker: PAPP-A and Free beta-hCG
- NT Scan: The first trimester screening programme offers a noninvasive option for the early detection of aneuploidy pregnancies. This screening is done by a combination of two biochemical markers i.e. serum free β-human chorionic gonadotrophin (free β-hCG) and pregnancy associated plasma protein A (PAPP-A), maternal age and fetal nuchal translucency (NT) thickness at 11 + 0–13 + 6 weeks of gestation. A beneficial consequence of screening is the early diagnosis or trisomies 21, 18 and 13. At 11 + 0–13 + 6 weeks, the relative prevalence of trisomies 18 and 13 to trisomy 21 are found to be one to three and one to seven, respectively. All three trisomies are associated with increased maternal age, increased fetal NT and decreased PAPP-A, but in trisomy 21 serum free β-hCG is increased whereas in trisomies 18 and 13 free β-hCG is decreased.
- NIPT: Non invasive prenatal testing/screening by cell free fetal DNA in maternal circulation
2. Second trimester Screening for Aneuploidy: Triple Screen or Triple maker or Quad Screen.
3. The Genetic Sonogram or Anomaly scan or Targeted imaging for Fetal Anomaly ( Target Scan)
HOW ACCURATE ARE THE SREENING METHODS?
These are the screening Test , shows the chance that a baby has a certain birth defect. The accuracy of a screening test is based on how often the test correctly finds a birth defect.
- The nuchal translucency test correctly finds Down syndrome in 64 to 70 out of 100 fetuses who have it. It misses Down syndrome in 30 to 36 out of 100 fetuses.
- First-trimester screening (nuchal translucency combined with the blood tests) correctly finds Down syndrome in 82 to 87 out of 100 fetuses who have it. This also means that these tests miss it in 13 to 18 out of 100 fetuses.
- The integrated test (first-trimester tests plus the quad screening in the second trimester) correctly finds Down syndrome in about 95 out of 100 fetuses who have it. This also means that the test misses Down syndrome in 5 out of 100 fetuses.
It’s possible that a screening test will be positive-meaning the test result is abnormal-but the baby doesn’t have the problem. This is called a false-positive test result. And it’s also possible that a screening may show that a baby doesn’t have a birth defect when he or she does have it. This is called a false-negative test result.
A false-positive result can cause stress and lead to unnecessary testing (such as chorionic villus sampling [CVS]). Many women who have a positive screening test result are actually carrying a healthy baby.