Prenatal screening for Down syndrome

Question 1

When is tier 1 screening done?

Answer 1

First trimester or second trimester according to duration of pregnancy

Question 2

What is done in the 1st trimester screening test? What is done on the same day? What is the detection rate of the 1st trimester screening test?

Answer 2

• Nuchal translucency test on 11-13 weeks + 6 days of gestation U/S
• Blood test will be performed on the same day to measure Pregnancy Associated Plasma Protein A (PAPP-A) and human chorionic gonadotropin (hCG) levels
• 1st trimester screening detects up to 90% of Down syndrome pregnancies

Question 3

What is done in 2nd trimester screening? What is the detection rate of the 2nd trimester screening test?

Answer 3

At 16-19 weeks, blood test for:
- AFP
- hCG
- uE3 (estriol)
- inhibin-A

2nd trimester screening test detects around 80% of Down syndrome pregnancies

Question 4

When will 1st tier screening test results be available?

Answer 4

2 weeks after the blood test

If result is low-risk, you will know your result at the next antenatal visit.

If result is high-risk, you will be informed and contacted to make an appointment for further counselling

Question 5

What are the options for further Ix if the 1st tier screening results are high-risk?

Answer 5

Non-invasive prenatal test (NIPT)

Diagnostic testing (chorionic villi sampling or amniocentesis)

Question 6

What is NIPT?

Answer 6

• Blood test based on the identification and counting of DNA fragments in maternal plasma originating from the placenta
• DNA from placenta usually represents foetal DNA
• NIPT provided by HA only includes detection of T21, T18, T13
• No report on foetal sex or sex chromosome / chromosomal abnormalities
• No risk of miscarriage as NIPT only required maternal blood sample
• Can detect 99.7% of T21, 97.8% T18, 95.8% T13
• Takes 10 days

Question 7

What is done if NIPT +ve?

Answer 7

Diagnostic test:
- Introduce needle under USG guidance into uterus to draw placental tissue or amniotic fluid for chromosome study
- There would be slight increased risk of miscarriage after either procedure compared to pregnancies that did not have procedures during the same gestational period
- Rapid chromosome test targetting T21, T18, T13
- Takes 2-3 days
- If rapid chromsome test yield normal results, HA will arrange chromosome microarray test to identify differences in amount of genetic material
- Imbalance in amount of genetic material may cause birth defects, delays in development and genetic syndromes
- Blood will be taken from parents if there is imbalance in genetic material

Question 8

What should we tell mothers about Down syndrome?

Answer 8

• Cardiac disease
• Intellectual disability
• Limited ability for self-care
• 1 in 700 babies have this
• The older the mother, the higher the risk of having Down syndrome baby (1 in 100 babies)
• Special socioenvironmental factors when raising baby with Down syndrome
• USG scan to look for foetal abnormalities
• First trimester screening 90% accurate
• Second trimester screening 80%, so book early!