Prenatal screening and diagnostic testing

Prenatal aneuploidy testing has two different jobs: screening (estimating chance) and diagnostic testing (giving a yes or no answer). Screening uses ultrasound (nuchal translucency), maternal serum markers and/or cell-free DNA; diagnostic testing examines fetal cells from chorionic villus sampling or amniocentesis. Every pregnant person should be offered both screening and diagnostic options after informed, documented discussion, and has the right to accept or decline any part of them. Cell-free DNA screening is the most sensitive and specific screening test for the common trisomies 21, 18 and 13 (detection rate about 99% for trisomy 21), but it remains a screen, not a diagnosis. Older combined first-trimester screening detects roughly 82 to 97% of trisomy 21, the quadruple (quad) screen about 81% and nuchal translucency alone about 70%, each at a 5% screen-positive rate. Nuchal translucency is measured at the 11 to 14 week scan; common thresholds used to prompt diagnostic testing are 3.0 mm (AWMF) or 3.5 mm (SOGC). Chorionic villus sampling (CVS) is performed from 11+0 to 13+6 weeks and amniocentesis from 15+0 weeks; in experienced hands the added miscarriage risk for each is likely under 0.5%. Regional services run to different timetables: NICE (United Kingdom) offers a scan at 11+2 to 14+1 weeks that can include aneuploidy screening, followed by a scan at 18+0 to 20+6 weeks; WHO recommends at least one ultrasound before 24 weeks. ^{[1][3][5][6][7]}

By the end of this page you should be able to:

1. Distinguish screening tests from diagnostic tests in prenatal care and explain what each can and cannot tell a patient.
2. Choose and time the main screening options (combined first-trimester screen, nuchal translucency, quad screen, cell-free DNA) and quote their approximate detection rates for trisomy 21.
3. Interpret a positive cell-free DNA result, including why the positive predictive value depends on maternal age, and explain why confirmatory diagnostic testing is required.
4. Plan and consent for CVS and amniocentesis at the right gestational ages, including their risks and the limits of confined placental mosaicism.
5. Communicate results, document informed decision making, and follow a stepwise pathway from booking through result disclosure and referral.
6. Recognise the common pitfalls, including screening after a vanishing twin, obesity, fetal fraction and decision making on screening results alone.

Prenatal screening and diagnostic testing exist to give pregnant people and their clinicians information early enough to make decisions. Because every option has limits, the goal is never a single "best test"; it is an informed choice that matches the person's wish for information with the test's strengths and weaknesses.^[2] All patients should be offered both screening and diagnostic tests, and all patients have the right to accept or decline testing after counseling.^[2]

Every pregnant person, regardless of age or baseline risk, should be offered information about testing, including those who conceive with donor oocytes or assisted reproduction and those who have had a transplant; these specific situations benefit from genetics referral because cfDNA interpretation changes.^[1] The person may want no testing at all, screening only, or diagnostic testing; there is no single correct option. A higher maternal age raises the prior risk of trisomy 21, 18 and 13, which raises the positive predictive value of any positive screen, but it does not by itself make cfDNA or any test mandatory.^[1]

Nuchal translucency (NT), the fluid-filled space at the back of the fetal neck seen on ultrasound, is measured with the Fetal Medicine Foundation protocol at a crown-rump length of 45 to 84 mm, usually 11+0 to 13+6 weeks.^[7] Combined with maternal serum free beta-hCG and PAPP-A (pregnancy-associated plasma protein A), this detects trisomy 21 with approximately 82 to 97% sensitivity at a 5% screen-positive rate, with the exact figure depending on gestation: 87%, 85% and 82% for scans performed at 11, 12 and 13 weeks respectively.^[1] Adding nasal bone assessment raises the detection rate to about 97% at a 5% screen-positive rate; adding an early serum AFP is laboratory-dependent. Ductus venosus and tricuspid flow Doppler are used by some guidelines as additional markers in intermediate-risk pregnancies.^[7]

NT alone detects about 70% of trisomy 21 at a 5% screen-positive rate and also surveys the fetal anatomy and detects multiple pregnancy.^[1] Increased NT is the most common first-trimester abnormal finding and prompts genetic evaluation even when the combined screen is not performed.^[7]

The quad screen measures maternal serum alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), unconjugated estriol (uE3) and dimeric inhibin A, and detects trisomy 21 in about 81% of pregnancies at a 5% screen-positive rate. No specialist ultrasound is required. It is the standard option when first-trimester screening was missed and a useful safety net.^[1]

When both first- and second-trimester samples are used, detection rises: integrated screening (nuchal translucency plus PAPP-A, then the quad screen) about 96%; serum-integrated (PAPP-A then quad) about 88%; stepwise sequential about 95%; contingent sequential about 88 to 94%.^[1] These take two samples and, except for stepwise screening, delay a result until the second trimester.

Cell-free DNA screening reads small fragments of placental DNA in maternal blood from 9 to 10 weeks to term. It is the most sensitive and specific screening test for trisomies 21, 18 and 13 (detection rate about 99% for trisomy 21, at a 2 to 4% screen-positive rate which includes test failures).^[1] The placental fraction is about 10 to 20% of circulating cell-free DNA, peaking between 10 and 21 weeks; most laboratories need a minimum fetal fraction of 2 to 4%.^[1] cfDNA may be reported at any gestational age after 9 to 10 weeks, which makes it invaluable when a person presents late or needs early information.

No single test is superior in every situation. The person's priorities (early result, highest detection, one sample only, wish to include twins, cost) guide the choice.^[2] An anatomical ultrasound at 18 to 20 weeks is recommended whatever screening route is chosen, because ultrasound detects structural findings that serum and cfDNA screens do not.^[1]

The decision points below are the ones a clinician uses to move from screening to diagnostic testing. Thresholds differ between guidelines and regions, so quote the guideline you follow.

Take the routine history, identify risk factors, and offer the infectious disease, sickle cell and thalassaemia, and fetal anomaly screening programmes, making clear that any part can be declined.^[3] Discuss timing and whereabouts of scans (NICE 11+2 to 14+1 weeks and 18+0 to 20+6 weeks), diet, medicines and any modifiable risks. Document the screening decisions and the plan for returning results.

Confirm viability, dates, number and chorionicity, and perform the anatomical survey. Measure the nuchal translucency and, where offered, free beta-hCG and PAPP-A. If nuchal translucency is increased (3.0 mm or 3.5 mm, depending on the guideline you follow) or other abnormalities appear, arrange genetic counselling and offer diagnostic testing. Where cfDNA has already been performed with a high-risk result, offer diagnostic testing. If a vanishing twin is found, see pitfalls.

Anomaly scan to look for structural malformations and to check placental location. A normal cfDNA or serum screen never replaces this scan. Abnormal findings here are an indication for diagnostic testing regardless of prior screening results.

Disclose promptly, ideally with a genetics professional or maternal-fetal medicine specialist. Explain that a positive screen raises the chance but does not diagnose. Offer diagnostic testing (CVS up to 13+6 weeks, otherwise amniocentesis from 15+0 weeks) and give time for questions. Avoid turning a positive screen into a management decision before confirmation: in one large series, about 6% of people with a positive cfDNA result underwent termination without confirmatory testing.^[1]

CVS collects placental tissue at 11+0 to 13+6 weeks; amniocentesis collects amniotic fluid from 15+0 weeks. The two carry a similar added miscarriage risk, likely below 0.5% in skilled hands, with published estimates for CVS ranging from 0.29% to 2%.^[5][6] For twins, the added risk is around 1%.^[5] CVS should not be done before 10+0 weeks because of a small but reported risk of limb defects at very early gestation; amniocentesis should not be done before 15+0 weeks.^[5] Risks otherwise include bleeding, infection and rupture of membranes. Send the sample for conventional karyotype, and for microarray when appropriate, and give results with counselling.

Offer non-directive counselling about the condition, prognosis, available interventions (including palliative neonatal care and termination where legal and appropriate), and referral to relevant specialists and support organisations. Document the decision and the care plan.

Refer to maternal-fetal medicine or clinical genetics when any of the following occur: an increased nuchal translucency (3.0 mm or 3.5 mm, depending on guideline); an abnormal cfDNA result; a nonreportable cfDNA result, especially with a low fetal fraction; multiple soft markers or structural anomalies on ultrasound; a family history of chromosomal rearrangement; a parental balanced translocation; a twin or higher-order pregnancy with discordant growth; or the person themselves asks for diagnostic testing. CVS or amniocentesis should be carried out in a unit with trained operators, ultrasound guidance and laboratory backup.^[5] In low-resource settings, a dating and anomaly scan and a basic combined first-trimester strategy, or at least a quad screen where ultrasound is limited, are better than no screening, and referral pathways to a centre that can perform invasive testing should be agreed in advance. WHO's guidance frames routine antenatal care as available in any healthcare or community setting, which includes making a clear plan for referral.^[4]

cfDNA is interpreted as a probability, not a result. The figures below come from SMFM (PMC13344640).

Because the prevalence of the common trisomies rises with maternal age, the positive predictive value rises with it; for trisomies 18 and 13 the PPV is much lower than for trisomy 21 at any age.^[1] Several factors lower cfDNA accuracy independent of the fetus: low fetal fraction (minimum 2 to 4%), maternal mosaicism or sex chromosome variation, a vanishing twin, maternal malignancy, confined placental mosaicism, and maternal autoimmune disease or low-molecular-weight heparin use. When the result is nonreportable, 0.03 to 11.1% of studies report this overall and an ACMG review of 31 studies estimated 0.85%.^[1]

CVS samples placental tissue, and confined placental mosaicism occurs in about 1 to 2% of pregnancies. On cfDNA, trisomy 16 usually leads to a third-trimester growth scan; trisomy 13 and 45,X need amniocentesis when the first-trimester ultrasound is normal but can use CVS when it is abnormal; trisomies 18 and 21 are confirmed by CVS. Multiple aneuploidies on cfDNA confer a 5 to 50% risk of maternal malignancy.^[1][6]

ACOG, SMFM and ACMG all stress that cfDNA is a screen, that no test is diagnostic before confirmatory testing, and that counselling covers the following points:^[1][2]

• The test is a screen, not a diagnosis; pregnancy decisions should not rest on cfDNA alone.
• Which conditions are tested and that cfDNA does not screen for all genetic conditions.
• Alternative screening and diagnostic options, tailored to risk, cost and the wish for information.
• The right to accept or decline any test.
• Test performance in singleton and twin pregnancy, including the possibility of false-positive, false-negative and inconclusive results.
• The possibility of unanticipated findings, including maternal chromosome variants or maternal conditions such as malignancy, and the need for follow-up.
• Cost, insurance and how results will be returned.
• Support and genetics referral when requested.
• Documentation of the informed decision and the plan for returning results.

RCOG (Green-top Guideline No. 8) adds that people considering CVS or amniocentesis need detailed counselling and pregnancy mapping by trained professionals, and care organised to national screening standards.^[5]

CVS ahead of 15 weeks gives an early, definitive fetal karyotype from placental tissue; amniocentesis is the route from 15 weeks. The physical risk of either in experienced hands is likely below 0.5% added miscarriage for singletons and around 1% for twins, with published CVS estimates between 0.29% and 2%.^[5][6] Both procedures also carry small risks of bleeding, infection and rupture of membranes, and there is no automatic indication for antibiotics or analgesia. The sample should be handled by a laboratory that can provide conventional karyotype and, where appropriate, microarray, with rapid preliminary results when clinically needed. A normal invasive result excludes the tested conditions; it does not exclude all structural or later-onset disease, which is why the mid-pregnancy anomaly scan still matters. Non-directive counselling after a confirmed diagnosis covers the condition, likely outcome, available interventions, termination options where legal and appropriate, and referral to relevant specialists and support organisations.

Document the screening offer, the information given, the person's decision (accept, decline, or decline a part), the test chosen and its limitations, and the plan for returning results, before testing.^[1] When a result is positive, give it in plain language, with the test's positive predictive value explained in the context of maternal age (for trisomy 21, 48% at age 20, 51% at 25, 79% at 35 and 93% at 40), and arrange confirmatory diagnostic testing. Record counselling, referrals, consent and the follow-up plan. Explain unexpected cfDNA findings plainly, including that cfDNA can reveal maternal chromosome differences or, rarely, maternal cancer; the latter has a 5 to 50% risk when cfDNA shows multiple aneuploidies or autosomal monosomy patterns, and warrants oncology work-up.^[1]

• Treating a positive screen as a diagnosis and making irreversible decisions before confirmatory testing.
• Ignoring a nonreportable cfDNA result: rates range from 0.03 to 11.1%, and around 0.85% in an ACMG review of 31 studies, with low fetal fraction more common in trisomy 13, 18 and triploidy and in obesity, early gestation and twin pregnancy. Repeat the test or move to diagnostic testing rather than reassuring without a result.^[1]
• Screening after a vanishing twin: cfDNA may read placental DNA from both twins; false-positive and false-negative rates change. Use a guideline that supports cfDNA in surviving twins or choose diagnostic testing.
• Obesity, which increases maternal DNA relative to fetal DNA, lowers fetal fraction; high maternal BMI is a common reason for a nonreportable cfDNA result.
• Maternal cell contamination and confined placental mosaicism (1 to 2% of CVS): CVS reflects placental tissue, not fetal tissue directly. When CVS shows mosaicism or an unexpected result, follow with amniocentesis. In trisomy 16 this may need a third-trimester growth scan.^[1][6]
• Measuring nuchal translucency outside the validated window (11+0 to 13+6 weeks, crown-rump length 45 to 84 mm) or by an untrained operator, which degrades accuracy.^[7]
• Missing a second-trimester anomaly scan because cfDNA was reassuring; cfDNA does not detect most structural anomalies.
• Calling cfDNA "NIPT" or "non-invasive prenatal testing" in counselling: the term suggests diagnosis and may minimise the person's attention to other screening and testing options.^[1]

Where resources are limited, prioritise a single first-trimester scan for dating and chorionicity, measure nuchal translucency if an operator is trained, and carry out a quad screen where ultrasound is unavailable; agree referral routes for diagnostic testing in advance. WHO frames routine antenatal care as available in any healthcare or community setting, which supports planning simple pathways.^[4] In multifetal pregnancies, nuchal translucency allows individual fetus assessment, but cfDNA is limited for twins and not recommended for higher-order multiples; the added miscarriage risk of CVS or amniocentesis in twins is around 1%.^[1][5] In pregnancies conceived with donor oocytes or after transplant, seek genetics advice before cfDNA because results may reflect the donor or the recipient.^[1]

• Screening (chance) and diagnostic testing (yes or no) are different; every pregnant person should be offered both and may accept or decline.
• cfDNA is the most sensitive and specific screen for trisomies 21, 18 and 13, with a detection rate of about 99% for trisomy 21 from 9 to 10 weeks to term.
• Combined first-trimester screening detects about 82 to 97% of trisomy 21, the quad screen about 81% and nuchal translucency alone about 70%, all at a 5% screen-positive rate.
• A positive screen is not a diagnosis; confirm with CVS or amniocentesis before any irreversible decision.
• CVS is performed from 11+0 to 13+6 weeks, amniocentesis from 15+0 weeks; in experienced hands the added miscarriage risk for each is probably below 0.5%, around 1% in twins.
• cfDNA positive predictive value rises with maternal age (trisomy 21 PPV 48% at 20 years, 79% at 35 years, 93% at 40 years); specificity exceeds 99.8%. ^[1]
• Nuchal translucency thresholds that prompt diagnostic work-up are around 3.0 mm (AWMF) or 3.5 mm (SOGC); ultrasound is measured only at 11+0 to 13+6 weeks.
• cfDNA nonreportable rates range from 0.03 to 11.1%, most often from low fetal fraction (minimum 2 to 4%); this is more common in trisomy 13, 18, triploidy and obesity and requires repeat testing or diagnostic referral.
• Confined placental mosaicism occurs in about 1 to 2% of CVS cases; mosaicism on CVS should be confirmed by amniocentesis.
• A second-trimester anomaly scan at 18+0 to 20+6 weeks (NICE) or the locally used window is needed whatever screening was done, because cfDNA does not detect most structural anomalies.
• Communication matters as much as the test: document the offer, the information, the decision and the plan; disclose positive screens promptly in plain language with counselling and confirmatory testing.

Prenatal screening connects with antenatal care, fetal anomaly ultrasound, Down's syndrome (trisomy 21), Edwards' syndrome (trisomy 18), Patau's syndrome (trisomy 13), Turner syndrome, genetic counselling, termination of pregnancy, multiple pregnancy, and invasive procedures in obstetrics. These are written as plain-text cross-references; link targets are added when the corresponding MedGrasp topics are verified.