# Fetal Aneuploidy Screening

> NEET-PG OBG notes on fetal aneuploidy screening covering first-trimester combined test, triple and quadruple markers, NT cut-offs and NIPT.

- Canonical URL: https://prepelephant.com/topics/neet-pg/obstetrics-and-gynaecology/screening-for-fetal-aneuploidy
- Exam / course: NEET-PG · Subject: Obstetrics and Gynaecology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Fetal Aneuploidy Screening", PrepElephant, https://prepelephant.com/topics/neet-pg/obstetrics-and-gynaecology/screening-for-fetal-aneuploidy

## Direct answer

Aneuploidy screening estimates the chance that a fetus has Down syndrome (trisomy 21), Edwards syndrome (trisomy 18) or an open neural tube defect, and it is offered to every pregnant woman irrespective of age. The first-trimester combined test at 11-13+6 weeks pairs nuchal translucency ultrasound with serum PAPP-A (low in trisomy 21) and free beta-hCG (high), detecting roughly 85-90 per cent of cases. The second-trimester quadruple test (alpha-fetoprotein, hCG, unconjugated oestriol and inhibin A) performs at about 80 per cent, and cell-free fetal DNA screening approaches 99 per cent for trisomy 21 — yet all remain screening, with chorionic villus sampling and amniocentesis the diagnostic tests.

## What you must remember

- Combined first-trimester test (11-13+6 weeks): nuchal translucency plus PAPP-A and free beta-hCG; a thickened nuchal translucency of 3.5 mm or more warrants referral and genetic counselling regardless of the serum risk.
- Additional first-trimester sonomarkers: absent nasal bone, reversed a-wave in the ductus venosus and tricuspid regurgitation raise the risk further.
- Down syndrome pattern: low PAPP-A with high free beta-hCG in the first trimester; in the quadruple test — low alpha-fetoprotein, high hCG, low unconjugated oestriol and high inhibin A.
- Trisomy 18 pattern: all markers low (PAPP-A, beta-hCG, AFP, oestriol, inhibin A) with an early growth-restricted fetus.
- Neural tube defect screening: raised maternal serum alpha-fetoprotein at 15-20 weeks, confirmed by detailed ultrasound and, when needed, amniotic fluid alpha-fetoprotein and acetylcholinesterase.
- Cell-free fetal DNA (NIPT): a screening test from about 10 weeks analysing placental DNA, highly sensitive for trisomy 21 with lower performance for trisomy 13, sex chromosome aneuploidy and twin pregnancies; not valid after organ transplantation, recent transfusion or maternal malignancy.
- Diagnostic tests: chorionic villus sampling at 10-13 weeks (transabdominal or transcervical) and amniocentesis from about 15-16 weeks; karyotype, rapid FISH or microarray; procedure-related miscarriage risk of the order of 0.5 per cent or less.

## Common confusion

Screening and diagnosis are repeatedly conflated. NIPT, the combined test and the quadruple test only modify a prior probability — a "high risk" result demands counselling and a diagnostic test, while a "low risk" result does not exclude aneuploidy. The second trap is marker direction: alpha-fetoprotein is raised in open neural tube defects and abdominal wall defects but low in Down syndrome; hCG behaves oppositely in the two aneuploidies — high in trisomy 21, low in trisomy 18. Writing the pattern grid for each condition prevents most errors.

## Exam-focused takeaway

The exam gives marker panels and asks which aneuploidy they suggest, or gives a nuchal translucency value and asks the next step. Window-based questions (11-13+6 weeks, 15-20 weeks), the 3.5 mm nuchal cut-off and the screening-versus-diagnostic hierarchy are frequent. Vignettes may ask which invasive test and when — chorionic villus sampling in the late first trimester versus amniocentesis after 15 weeks — including its role in neural tube defect follow-up.

## Frequently asked questions

### What constitutes the first-trimester combined test?

Nuchal translucency measurement with serum PAPP-A and free beta-hCG between 11 and 13+6 weeks, reported as an individualised risk.

### How do triple and quadruple tests differ?

The triple test measures AFP, hCG and oestriol at 15-20 weeks; the quadruple test adds inhibin A, improving Down syndrome detection.

### What is the marker pattern in Down syndrome?

Low AFP and oestriol with high hCG and inhibin A on the quadruple test; in the first trimester, low PAPP-A with high free beta-hCG.

### Is NIPT a diagnostic test?

No — it is a highly sensitive screening test; positive results must be confirmed by chorionic villus sampling or amniocentesis before irreversible decisions.

### When are the invasive tests performed?

Chorionic villus sampling at 10-13 weeks and amniocentesis from about 15-16 weeks, each with a small procedure-related miscarriage risk.