[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Biochemistry](https://durgical.com/en/biochemistry/) › Preconception and prenatal genetics: screening, diagnosis and fetal therapy

# Preconception and prenatal genetics: screening, diagnosis and fetal therapy

Genetic options before and during pregnancy: carrier screening, embryo testing, screening tests that estimate risk, and diagnostic tests that give an answer.

Updated Nov 13, 2026

## Key facts

- Carrier screening before pregnancy finds couples who could have a child with a recessive or X-linked disorder (cystic fibrosis, spinal muscular atrophy, haemoglobinopathies, fragile X); expanded panels test hundreds of conditions at once.
- Preimplantation genetic testing examines a few cells from IVF embryos: for a known familial variant (PGT-M), a parental structural rearrangement (PGT-SR) or aneuploidy (PGT-A).
- Screening tests estimate risk; they do not diagnose. They include first-trimester combined screening (nuchal translucency with maternal serum markers), second-trimester serum markers, the anomaly ultrasound and cell-free DNA.
- Cell-free DNA screening detects trisomies 21, 18 and 13 very sensitively, but the DNA is placental, so a positive result needs confirmation by a diagnostic test.
- Diagnostic procedures: chorionic villus sampling (from about 10–13 weeks) and amniocentesis (from about 15 weeks), each with a small risk of miscarriage.

## In detail

- Laboratory studies: rapid aneuploidy tests (FISH or quantitative PCR), karyotype, chromosomal microarray (recommended when ultrasound shows a structural anomaly) and, increasingly, exome sequencing for anomalous fetuses.
- The positive predictive value of any screening test falls when the condition is rare, as in younger women.
- Confined placental mosaicism can make cell-free DNA or a chorionic villus result differ from the fetus.
- Fetal interventions include intrauterine transfusion for fetal anaemia, laser treatment for twin-to-twin transfusion and prenatal repair of myelomeningocele in selected cases.
- Counselling is non-directive and covers what each test can and cannot detect, the meaning of results and all options.

**Also called**
Carrier screening and preimplantation testing, Cell-free DNA, CVS and amniocentesis

## Clinical notes

- A high-risk cell-free DNA result should not lead to decisions about the pregnancy without diagnostic confirmation.
- When both partners carry a recessive variant, options include prenatal diagnosis and preimplantation testing.

## Test yourself

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

## Why must a positive cell-free DNA screen be confirmed?

It is a screening test on mostly placental DNA; false positives occur, especially from confined placental mosaicism.

## What is the difference between PGT-M and PGT-A?

PGT-M tests embryos for a known familial single-gene variant; PGT-A screens embryos for whole-chromosome aneuploidy.

## Which test is recommended when a fetal structural anomaly is seen on ultrasound?

A chromosomal microarray on a diagnostic sample, often followed by exome sequencing.

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## Connections

## Related conditions 5

- [Down syndrome (trisomy 21)](https://durgical.com/en/conditions/down-syndrome/)
- [Cystic fibrosis](https://durgical.com/en/conditions/cystic-fibrosis/)
- [Spinal muscular atrophy](https://durgical.com/en/conditions/spinal-muscular-atrophy/)
- [Sickle cell disease](https://durgical.com/en/conditions/sickle-cell-disease/)
- [Thalassemias (α and β)](https://durgical.com/en/conditions/thalassemias/)

## Related 5

- [Genetic tests: what each one finds](https://durgical.com/en/biochemistry/topics/genetic-tests-what-each-one-finds/)
- [Genetic counselling and risk assessment](https://durgical.com/en/biochemistry/topics/genetic-counselling-and-risk-assessment/)
- [Population genetics: Hardy–Weinberg and carrier frequency](https://durgical.com/en/biochemistry/topics/population-genetics-hardy-weinberg-and-carrier-frequency/)
- [FISH (fluorescence in situ hybridization)](https://durgical.com/en/biochemistry/topics/fish/)
- [DNA microarrays](https://durgical.com/en/biochemistry/topics/dna-microarrays/)

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Carrier Screening](https://www.genome.gov/genetics-glossary/Carrier-Screening) (National Human Genome Research Institute (NHGRI), 2026); [What are the different types of genetic tests?: MedlinePlus Genetics](https://medlineplus.gov/genetics/understanding/testing/types/) (MedlinePlus, US National Library of Medicine, 2026); [Amniocentesis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003921.htm) (MedlinePlus, US National Library of Medicine, 2026); [Chorionic villus sampling: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003406.htm) (MedlinePlus, US National Library of Medicine, 2026).

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/topics/preconception-and-prenatal-genetics-screening-diagnosis-and-fetal-therapy/