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.
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.
Connections
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Carrier Screening (National Human Genome Research Institute (NHGRI), 2026); What are the different types of genetic tests?: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Amniocentesis: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Chorionic villus sampling: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.