Embryology · Teratology and prenatal diagnosis
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
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 | 11–14 | Risk of trisomies 21, 18 and 13 |
| Cell-free DNA | From 10 | Sensitive screen for trisomies and sex chromosomes |
| Quad screen (AFP, hCG, estriol, inhibin A) | 15–22 | Risk of trisomies and neural tube defects |
| Anatomy scan | 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.
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.
Related
- Meiosis and nondisjunction: where trisomies come from
- Gestational age vs fertilization age: how pregnancy is dated
- Neural tube defects: anencephaly, spina bifida and encephalocele
- Omphalocele and gastroschisis
- Amniotic fluid: polyhydramnios and oligohydramnios
- Placenta: structure, circulation and functions
- Trophoblast and hCG; gestational trophoblastic disease
- Folic acid and neural tube defect prevention
- Limb defects: amelia, meromelia, polydactyly and syndactyly
Related conditions
- Down syndrome (trisomy 21)
- Spina bifida (myelomeningocele)
- Turner syndrome
- Klinefelter syndrome (47,XXY)
- DiGeorge syndrome (22q11.2 deletion syndrome)
- Rh alloimmunisation in pregnancy
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 (StatPearls Publishing (NCBI Bookshelf), 2023); Chorionic Villus Sampling (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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Representative model, for educational purposes only.