Synopsis
Explain England's current trisomy screening sequence, interpret chance results accurately and support timely choice between no further testing, NIPT and invasive diagnosis.
- The combined test uses maternal age, nuchal translucency, pregnancy-associated plasma protein A and free beta-hCG when crown-rump length is 45.0 to 84.0 mm, approximately 11+2 to 14+1 weeks.
- A combined test estimates chance of trisomy 21, trisomy 18 and trisomy 13; it is not a diagnostic fetal karyotype.
- If nuchal translucency cannot be obtained or crown-rump length exceeds 84.0 mm, offer the quadruple test when head circumference is 101.0 to 172.0 mm and blood is taken from 14+2 to 20+0 weeks.
- The 2026 FASP handbook describes quadruple screening for trisomy 21 and trisomy 18 when T18 implementation is active; confirm the current laboratory pathway, and do not claim that quadruple testing screens for trisomy 13.
- A chance from 1 in 2 through 1 in 150 at term is higher chance; 1 in 151 or lower is lower chance. These categories are probability thresholds, not affected or unaffected diagnoses.
- After a higher-chance combined or quadruple result, offer an informed choice of no further testing, NHS NIPT or invasive prenatal diagnosis.
- NIPT analyses placental cell-free DNA for trisomy 21, 18 and 13 in this NHS pathway; lower chance reduces risk, while higher chance or no result needs specialist discussion and may lead to CVS or amniocentesis.
- CVS is usually performed from 11+0 to 13+6 weeks and not before 10+0; amniocentesis is performed from 15+0 weeks. In skilled hands, additional procedure-related miscarriage risk is likely below 0.5% in singleton pregnancy.
Key red flags
Nuchal translucency of 3.5 mm or more requires immediate referral through the local fetal-medicine pathway without waiting for the calculated combined-test chance.
A major structural anomaly or several markers may indicate a chromosomal or genetic disorder outside the standard trisomy screen and needs specialist diagnostic discussion.
A higher-chance NIPT result is not diagnostic and should be confirmed by an appropriately selected invasive test before an irreversible pregnancy decision.
A no-result NIPT can be associated with low fetal fraction, placental dysfunction or aneuploidy and needs active counselling rather than being reported as low chance.
Persistent pain, heavy bleeding, fluid loss, fever or feeling unwell after CVS or amniocentesis requires urgent maternity or fetal-medicine contact.
A twin, vanished-twin or placental mosaicism context can complicate screening attribution and requires specialist interpretation of what the result applies to.
Investigation priorities
Estimate chance of trisomy 21, 18 and 13 using maternal age, serum markers and nuchal translucency.
Management branches
The patient chooses trisomy screening during the first or second trimester.
- Use crown-rump length to determine combined-test eligibility, obtain valid nuchal translucency and blood markers, and refer immediately when NT is at least 3.5 mm.
- When combined testing is not possible, check head circumference and the 14+2-to-20+0 sampling window before offering the quadruple test and explain its narrower or implementation-dependent scope.
- Communicate the numerical chance, test limitations and failsafe action in the patient's preferred format, recording separate choices for the conditions offered.