Antenatal Screening for Down’s Syndrome
Key Points ⚡
- Down’s syndrome (Trisomy 21): most common chromosomal abnormality; incidence 1 in 377 babies in England (2020).
- Associated with delayed milestones, congenital heart defects, GI anomalies.
- Screening is part of NHS Fetal Anomaly Screening Programme (FASP).
- Maternal age increases risk significantly (20 years: 1/1500; 30 years: 1/900; 40 years: 1/100).
Screening Tests
Combined Test (10+0 to 14+1 weeks gestation)
- Combines maternal age, serum markers, and ultrasound:
- Free beta-hCG: increased in Down’s syndrome.
- PAPP-A: decreased in Down’s syndrome.
- Nuchal translucency (NT): increased thickness associated with Down’s syndrome.
- Crown-rump length (CRL) used to confirm eligibility (45.0-84.0 mm).
- If NT cannot be measured, may use quadruple test or alternate screening per guidelines.
Quadruple Test (14+2 to 20+0 weeks gestation)
- Used if NT not measurable or late presentation.
- Measures:
- AFP (low), bhCG (high), inhibin-A (high), unconjugated estriol uE3 (low).
- Less accurate than combined test.
Screening Results
- High chance: ≥ 1 in 150 — proceed to NIPT or diagnostic testing.
- Low chance: ≤ 1 in 151 — routine care.
Non-Invasive Prenatal Testing (NIPT)
- Blood test assessing placental cell-free fetal DNA from 10 weeks onwards.
- High detection rate (up to 99% for trisomy 21).
- Considered screening, positive results require confirmatory diagnostic tests.
- Not suitable for multiple pregnancies > twins, maternal malignancy, recent transfusion/transplant.
Diagnostic Tests
- Chorionic Villus Sampling (CVS): 11-14 weeks; ultrasound-guided placental tissue sampling.
- Amniocentesis: >15 weeks; sampling of amniotic fluid.
- Both carry ~1 in 200 miscarriage risk.
- Rh-negative women receive anti-D immunoglobulin post-procedure.
Anomaly Scan
- Performed at 18+0 to 20+6 weeks to detect 11 physical conditions including some linked to Down’s syndrome (e.g., heart defects).
- Not all trisomy 21 cases have detectable anomalies on scan.
Special Considerations for Twin Pregnancies
- Combined test preferred.
- NIPT emerging but more complex.
- Higher miscarriage risk with invasive diagnostics.
Post-Test Counseling and Management
- Positive diagnosis: support from obstetrics, genetic counseling, pediatric consultation.
- Women choose to continue or terminate pregnancy; specialist support provided accordingly.
References
- National Congenital Anomaly and Rare Disease Registration Service, 2023
- Geeky Medics, Down's Syndrome, 2022
- Public Health England, NHS Fetal Anomaly Screening Programme, 2023
- NHS England Screening Guidelines, 2023
- Palomaki GE et al., Meta-Analysis of Down Syndrome Markers
- Stergiotou et al., CVS Practical Guide, 2016
- Kalousek and Vekemans, Placental Mosaicism
- Salomon et al., Miscarriage Risk Following CVS and Amniocentesis, 2019
- Khalil et al., NIPT in Twin Pregnancies, 2021
- Amniotic Fluid Embolism
- Antepartum Haemorrhage (APH)
- Breech Presentation
- Caesarean Section
- Cord Prolapse