Down’s Syndrome
NICE issued updated guidelines on antenatal care in 2021 including advice on screening for Down's syndrome
- the combined test is now standard
- these tests should be done between 11 - 13+6 weeks
- nuchal translucency measurement + serum B-HCG + pregnancy-associated plasma protein A (PAPP-A)
- Down's syndrome is suggested by ↑ HCG, ↓ PAPP-A, thickened nuchal translucency
- trisomy 18 (Edward syndrome) and 13 (Patau syndrome) give similar results but the hCG tends to lower
- quadruple test
- if women book later in pregnancy the quadruple test should be offered between 15 - 20 weeks
- quadruple test: alpha-fetoprotein, unconjugated oestriol, human chorionic gonadotrophin and inhibin A
Interpreting the results of quadrapule tests
|
Alpha-fetoprotein |
Unconjugated oestriol |
Human chorionic gonadotrophin |
Inhibin A |
| Down's syndrome |
↓ |
↓ |
↑ |
↑ |
| Edward's syndrome |
↓ |
↓ |
↓ |
↔ |
| Neural tube defects |
↑ |
↔ |
↔ |
↔ |
Results of combined or quadruple tests
Both the combined and quadruple tests return either a 'lower chance' or 'higher chance' result
- 'lower chance': 1 in 150 chance or more e.g. 1 in 300
- 'higher chance': 1 in 150 chance or less e.g. 1 in 100
Next Step →
Non-invasive prenatal screening test (NIPT)
If a woman has a 'higher chance' results she will be offered a second screening test (NIPT) or a diagnostic test (e.g. amniocentesis or chorionic villus sampling (CVS). Given the non-invasive nature of NIPT and extremely high sensitivity and specificity, it is likely this will be the preferred choice for the vast majority of women.
NIPT
- analyses small DNA fragments that circulate in the blood of a pregnant woman (cell free fetal DNA, cffDNA)
- cffDNA derives from placental cells and is usually identical to fetal DNA
- analysis of cffDNA allows for the early detection of certain chromosomal abnormalities
- sensitivity and specificity are very high for trisomy 21 (>99%) and similarly high for other chromosomal abnormalities