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Prenatal screening and diagnosis
Screening tests (nuchal translucency, biochemical markers, cell-free fetal DNA) only estimate risk; only an invasive test — chorionic villus sampling or amniocentesis — gives a diagnosis, so every positive screen is confirmed by a diagnostic test before a decision.
Why it matters in the exam
Exam questions test the timetable and the interpretation. The first-trimester combined screen at 11+0 to 13+6 weeks combines nuchal translucency, PAPP-A and β-hCG with maternal age; in Down syndrome PAPP-A is low and β-hCG high. A nuchal translucency of 3.5 mm or more calls for an offer of invasive testing with a chromosomal microarray and fetal echocardiography, even when the calculated risk is low. The second-trimester quadruple test (15–20 weeks) measures AFP, hCG, oestriol and inhibin A; in Down syndrome AFP and oestriol are low and hCG and inhibin high, and in trisomy 18 all are low.
A raised AFP alone points to a neural tube defect, an abdominal wall defect, multiple pregnancy, wrong dates or fetal death. Cell-free fetal DNA testing (NIPT) is possible from 10 weeks, and its sensitivity for trisomy 21 is very high, but it is still a screening test: a positive result may come from confined placental mosaicism, a vanished twin or even a maternal tumour. The diagnostic tests: chorionic villus sampling (CVS) at 10 to 13+6 weeks, and amniocentesis from 15 weeks; the miscarriage risk of both is low. When there is a structural anomaly on ultrasound, the recommended test is a chromosomal microarray. In Israel a national carrier-screening programme by ancestry is also offered.
Key points
- Combined screen: 11+0 to 13+6; Down: low PAPP-A, high β-hCG.
- Nuchal translucency 3.5 mm or more → invasive work-up and echocardiography.
- Quadruple test: Down — low AFP and oestriol, high hCG and inhibin.
- NIPT from 10 weeks — screening only.
- CVS 10 to 13+6; amniocentesis from 15 weeks; structural anomaly → microarray.
Summary table
| Test | Timing | What is measured | Interpretation in Down syndrome or note |
|---|---|---|---|
| First-trimester combined screen | 11+0 to 13+6 | Nuchal translucency, PAPP-A, β-hCG, age | Low PAPP-A, high β-hCG, increased translucency |
| Increased nuchal translucency | 3.5 mm or more | — | Offer invasive testing with microarray and fetal echocardiography |
| Quadruple test | 15 to 20 | AFP, hCG, oestriol, inhibin A | Down: low AFP and oestriol, high hCG and inhibin; trisomy 18: all low |
| Raised AFP | 15 to 20 | — | Neural tube defect, abdominal wall defect, twins, wrong dates |
| NIPT (cell-free fetal DNA) | From 10 weeks | Placental DNA in maternal blood | Screening only; positive → confirm by invasive testing |
| CVS / amniocentesis | CVS 10 to 13+6; amniocentesis from 15 weeks | Karyotype, microarray | Diagnostic; low miscarriage risk |
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Sources
- ACOG Practice Bulletin No. 226, 2020 — Screening for Fetal Chromosomal Abnormalities
- ACOG Practice Bulletin No. 162, 2016 — Prenatal Diagnostic Testing for Genetic Disorders
- ISUOG Practice Guidelines, 2013, updated 2023 — Performance of first-trimester fetal ultrasound scan
- Williams Obstetrics, 26th edition (2022) — Prenatal Diagnosis
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.