Prenatal Screening Pathology
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Direct answer
Screening tests estimate risk; diagnostic tests establish it — the first sentence of every antenatal-testing answer. First-trimester combined screening pairs nuchal translucency on ultrasound (11-13 weeks 6 days, crown-rump 45-84 millimetres) with serum PAPP-A (low) and free beta-hCG (high) — the double marker — detecting most Down syndrome pregnancies; the second-trimester triple test (AFP, unconjugated oestriol, hCG) and the quadruple (adding inhibin A) cover those who book late. Cell-free fetal DNA (NIPT) screens at above 99 per cent sensitivity for trisomy 21 from ten weeks but remains a screening test; definitive diagnosis requires chorionic villus sampling at 11-14 weeks or amniocentesis after 15 weeks with karyotype or chromosomal microarray. In India, all of it operates under the PCPNDT Act, 1994, which prohibits determination and disclosure of fetal sex.
What you must remember
- Marker patterns for trisomy 21: low AFP, low unconjugated oestriol, high hCG, high inhibin A — a four-analyte melody worth memorising; trisomy 18 shows all four analytes low.
- First-trimester package: nuchal translucency above 3.5 millimetres raises aneuploidy risk and, if karyotype is normal, congenital heart disease risk justifying fetal echocardiography; nasal bone, ductus venosus flow and tricuspid regurgitation add discrimination.
- Open neural tube defect: maternal serum AFP high, with acetylcholinesterase in amniotic fluid confirming an open defect; the anomaly scan closes the loop.
- Diagnostic tests with their windows and risks: chorionic villus sampling at 11-14 weeks (placental karyotype, earlier result, small fetoplacental mosaicism risk, procedure-related loss around 1-2 per cent); amniocentesis after 15 weeks (fetal cells from amniotic fluid, loss under 1 per cent); both now often coupled with rapid FISH or QF-PCR and chromosomal microarray, which also detects submicroscopic copy-number changes.
- NIPT caveats: a screening test needing diagnostic confirmation; results are confounded by placental mosaicism, vanishing twin, maternal malignancy and low fetal fraction (high BMI, early sampling); it screens 21, 18, 13 and sex-chromosome aneuploidy best.
- Thalassaemia screening — the Indian dimension: in a country with a large beta-thalassaemia burden, carrier screening by HPLC (haemoglobin A2 of 3.5 per cent or more) for couples, with prenatal diagnosis by CVS for at-risk pairs, is routine genetic-prevention practice — more relevant to Indian exams than any Western panel.
- The PCPNDT Act, 1994: regulates genetic clinics and prohibits sex determination and disclosure to combat female foeticide — the legal frame within which every Indian prenatal test is performed; a favoured medico-social viva point.
Interpreting a screen-positive triple test
A 27-year-old at 17 weeks has a triple test reporting a Down syndrome risk of 1 in 150; she is anxious and eight weeks from her anomaly scan. Step one is validation: was the sample dated correctly, and does the dating scan agree with her dates? A misdated pregnancy is the commonest source of false positives. Step two interprets the pattern, not the ratio: low AFP and oestriol with high hCG raise trisomy 21 risk; a very high AFP redirects the search to open neural tube defects or abdominal wall defects, with acetylcholinesterase in amniotic fluid as the discriminator. Step three offers choice — NIPT as a secondary screen, or straight to amniocentesis for a definitive karyotype with microarray. Step four supports the result whatever it is: a negative NIPT reduces risk substantially but does not remove the need for the anomaly scan; a positive report makes diagnostic testing mandatory. The architecture — validate, interpret pattern, offer tiered testing, counsel — is the same for every antenatal screen.
Where students slip
Calling NIPT diagnostic loses the mark every time; it is a highly sensitive screen on placental DNA, and discordance with the fetus, though rare, is real. The second error is reading the triple test as one number: the pattern of its four (or three) analytes points to different conditions, and pattern-reading is the examiner's actual question. Remember timing windows — CVS before amniocentesis in the calendar, both with FISH for rapid aneuploidy. And the PCPNDT answer must be precise: the Act prohibits sex determination and disclosure; it does not prohibit legitimate aneuploidy or metabolic testing.
Frequently asked questions
What constitutes first-trimester combined screening?
Nuchal translucency ultrasound at 11-13 weeks 6 days plus serum PAPP-A and free beta-hCG — the double marker — with detection of most trisomy 21 pregnancies.
Which marker pattern suggests open neural tube defect?
Elevated maternal serum alpha-fetoprotein, with acetylcholinesterase in amniotic fluid supporting an open lesion on confirmative testing.
Why is NIPT not diagnostic?
It analyses placental cell-free DNA, which can disagree with the fetal genome through confined placental mosaicism or maternal factors; abnormal results need CVS or amniocentesis.
When are chorionic villus sampling and amniocentesis performed?
CVS at 11-14 weeks on placental tissue; amniocentesis after 15 weeks on amniotic-fluid fetal cells — both with procedure-related miscarriage risk around or below 1-2 per cent.
What does the PCPNDT Act regulate?
It prohibits prenatal sex determination and disclosure in India, registering and monitoring genetic laboratories to counter female foeticide.