Question

How will you do antenatal screening of Down syndrome?

Hint:

The combination of nuchal translucency measurement, maternal serum markers, and non-invasive prenatal testing provides the most comprehensive screening for Down syndrome in pregnancy.

Answer 1

Step 1: Antenatal Screening for Down Syndrome.
Antenatal screening for Down syndrome aims to identify pregnancies at high risk of having a baby with Down syndrome (trisomy 21). The screening process involves a combination of maternal blood tests and ultrasound scans. The screening tests are usually done in the first trimester (11-13 weeks) and the second trimester (15-20 weeks). The main methods include:
(1) First Trimester Screening:
- Nuchal Translucency (NT) Measurement:
- An ultrasound scan is used to measure the nuchal translucency, which is the fluid-filled space at the back of the fetus' neck. An increased NT thickness can indicate a higher risk of Down syndrome.
- Maternal Serum Screening (PAPP-A and free \(\beta\)-hCG):
- Blood tests to measure the pregnancy-associated plasma protein-A (PAPP- and free beta-human chorionic gonadotropin (\(\beta\)-hCG) levels. Lower levels of PAPP-A and higher levels of free \(\beta\)-hCG are associated with an increased risk of Down syndrome.
- Combined Screening Test:
- The NT measurement along with PAPP-A and free \(\beta\)-hCG levels are combined to calculate the risk score for Down syndrome.
(2) Second Trimester Screening (Quadruple Test):
- Quadruple Test:
- This involves measuring four maternal serum markers: alpha-fetoprotein (AFP), estriol, inhibin-A, and beta-hCG. The levels of these markers are analyzed to assess the risk of Down syndrome. Abnormal levels are associated with an increased risk.
- Triple Test:
- Similar to the quadruple test but omits inhibin-A. It is used less frequently today because the quadruple test offers a more accurate risk assessment.
(3) Non-Invasive Prenatal Testing (NIPT):
- Cell-free DNA (cfDN Testing:
- A blood test from the mother that analyzes small fragments of fetal DNA circulating in her blood. This test has a high detection rate for Down syndrome and can be performed as early as 10 weeks of gestation. NIPT is more accurate than the traditional screening methods, and it can detect trisomy 21 with a higher sensitivity and specificity.
(4) Diagnostic Testing (if screening is positive):
- If a woman has a high-risk screening result, diagnostic tests such as chorionic villus sampling (CVS) or amniocentesis may be offered to confirm the diagnosis. These tests involve taking samples from the placenta or amniotic fluid and have a higher accuracy than screening tests. However, they carry a small risk of miscarriage.