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[Embryology](https://durgical.com/en/embryology/) · [Teratology and prenatal diagnosis](https://durgical.com/en/embryology/#a-teratology)

# Prenatal diagnosis: ultrasound, serum screening, cell-free DNA, CVS, amniocentesis

- Topic

The pregnancy tests that estimate the risk of an anomaly (screening) and those that confirm it (diagnosis), with their weeks.

Updated 2026-11-01

## Key points

- Screening estimates risk; diagnosis confirms. A positive screen is confirmed by chorionic villus sampling or amniocentesis.
- First trimester (weeks 11–14): nuchal translucency + PAPP-A + free β-hCG.
- Quad screen (weeks 15–22): AFP, hCG, unconjugated estriol and inhibin A. Trisomy 21: low AFP and estriol, high hCG and inhibin A.
- High maternal AFP: open neural tube defects, gastroschisis or omphalocele, multiple pregnancy or wrong dates.
- Cell-free DNA from week 10: comes from placental trophoblast; the most sensitive screen, but not diagnostic.

## What it is

Prenatal diagnosis combines screening tests, which estimate a risk, with diagnostic tests, which confirm or rule out an anomaly. Ultrasound dates the pregnancy and finds malformations; maternal serum markers and cell-free DNA estimate the risk of aneuploidy; chorionic villus sampling and amniocentesis obtain fetal cells for genetic diagnosis. On this page weeks are gestational (from the last menstrual period).

## Key facts

- Gestational weeks 5–10: crown-rump length (CRL) is the most accurate dating measure.
- Weeks 18–22: anatomy (anomaly) scan.
- Trisomy 18: low AFP, estriol and hCG.
- In open neural tube defects amniotic fluid AFP and acetylcholinesterase are raised.
- Chorionic villus sampling (CVS): weeks 10–13; amniocentesis: usually from week 15.

## Prenatal tests: when and what for

| Test | Weeks | What it gives |
| --- | --- | --- |
| Early ultrasound (CRL) | 5–10 | Exact gestational age, number of embryos |
| [Nuchal translucency + PAPP-A + β-hCG](https://durgical.com/en/conditions/down-syndrome/) | 11–14 | Risk of trisomies 21, 18 and 13 |
| [Cell-free DNA](https://durgical.com/en/conditions/down-syndrome/) | From 10 | Sensitive screen for trisomies and sex chromosomes |
| [Quad screen (AFP, hCG, estriol, inhibin A)](https://durgical.com/en/conditions/down-syndrome/) | 15–22 | Risk of trisomies and neural tube defects |
| [Anatomy scan](https://durgical.com/en/conditions/spina-bifida/) | 18–22 | Neural tube, heart, abdominal wall and facial defects |
| Chorionic villus sampling | 10–13 | Early chromosomal and genetic diagnosis |
| Amniocentesis | From 15 | Karyotype, genetic tests, AFP and acetylcholinesterase |

## Markers

- AFP
- β-hCG
- PAPP-A
- Estriol
- Inhibin A
- Acetylcholinesterase

## Clinical notes

- Spina bifida on ultrasound: the 'lemon' sign (flattened forehead) and the 'banana' sign (squeezed cerebellum).
- Increased nuchal translucency points to aneuploidy and also to congenital heart disease.
- Decisions after a result are made with genetic counselling and the treating team.

## In the lab

- Cell-free DNA can give a 'no call' result when the fetal fraction is low.
- CVS can show confined placental mosaicism, which is clarified by amniocentesis.
- CVS done before week 10 was linked to limb reduction defects.

## Memory aid

In trisomy 21 'the placenta's hormones go up (hCG, inhibin) and the baby's go down (AFP, estriol)'.

## Test yourself

Think of the answer, then tap the question to see it.

## Which quad screen pattern suggests trisomy 21?

Low AFP and estriol with high hCG and inhibin A.

## Why is cell-free DNA not a diagnostic test?

It comes from placental trophoblast, which may not match the fetus; a positive result is confirmed with CVS or amniocentesis.

## What raises maternal serum AFP?

Open neural tube defects, abdominal wall defects, multiple pregnancy and a miscalculated gestational age.

## In which weeks are chorionic villus sampling and amniocentesis done?

CVS between weeks 10 and 13; amniocentesis usually from gestational week 15.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/embryology/prenatal-diagnosis-ultrasound-serum-screening-cell-free-dna-cvs-amniocentesis/)

## Related

- [Meiosis and nondisjunction: where trisomies come from](https://durgical.com/en/embryology/meiosis-and-nondisjunction-where-trisomies-come-from/)
- [Gestational age vs fertilization age: how pregnancy is dated](https://durgical.com/en/embryology/gestational-age-vs-fertilization-age-how-pregnancy-is-dated/)
- [Neural tube defects: anencephaly, spina bifida and encephalocele](https://durgical.com/en/embryology/neural-tube-defects-anencephaly-spina-bifida-and-encephalocele/)
- [Omphalocele and gastroschisis](https://durgical.com/en/embryology/omphalocele-and-gastroschisis/)
- [Amniotic fluid: polyhydramnios and oligohydramnios](https://durgical.com/en/embryology/amniotic-fluid-polyhydramnios-and-oligohydramnios/)
- [Placenta: structure, circulation and functions](https://durgical.com/en/embryology/placenta-structure-circulation-and-functions/)
- [Trophoblast and hCG; gestational trophoblastic disease](https://durgical.com/en/embryology/trophoblast-and-hcg-gestational-trophoblastic-disease/)
- [Folic acid and neural tube defect prevention](https://durgical.com/en/embryology/folic-acid-and-neural-tube-defect-prevention/)
- [Limb defects: amelia, meromelia, polydactyly and syndactyly](https://durgical.com/en/embryology/limb-defects-amelia-meromelia-polydactyly-and-syndactyly/)

## Related conditions

- [Down syndrome (trisomy 21)](https://durgical.com/en/conditions/down-syndrome/)
- [Spina bifida (myelomeningocele)](https://durgical.com/en/conditions/spina-bifida/)
- [Turner syndrome](https://durgical.com/en/conditions/turner-syndrome/)
- [Klinefelter syndrome (47,XXY)](https://durgical.com/en/conditions/klinefelter-syndrome/)
- [DiGeorge syndrome (22q11.2 deletion syndrome)](https://durgical.com/en/conditions/digeorge-syndrome/)
- [Rh alloimmunisation in pregnancy](https://durgical.com/en/conditions/rh-alloimmunisation-in-pregnancy/)

[← All of embryology](https://durgical.com/en/embryology/)

Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.

Sources: [Prenatal Genetic Screening](https://www.ncbi.nlm.nih.gov/books/NBK557702/) (StatPearls Publishing (NCBI Bookshelf), 2023); [Chorionic Villus Sampling](https://www.ncbi.nlm.nih.gov/books/NBK563301/) (StatPearls Publishing (NCBI Bookshelf), 2023); Langman's Medical Embryology, 13th ed., ch. 9: Birth Defects and Prenatal Diagnosis (Wolters Kluwer, 2015).

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Source: https://durgical.com/en/embryology/prenatal-diagnosis-ultrasound-serum-screening-cell-free-dna-cvs-amniocentesis/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/embryology/prenatal-diagnosis-ultrasound-serum-screening-cell-free-dna-cvs-amniocentesis/