Clinical overview
Down syndrome (trisomy 21) is the commonest autosomal aneuploidy compatible with survival to term and the leading genetic cause of intellectual disability. For the registrar, the examinable challenge is rarely the karyotype itself — it is the counselling: how to offer screening so that a woman makes an informed, autonomous choice; how to interpret a screening result as a probability rather than a verdict; how to discuss diagnostic testing and its procedure-related risks; and how to do all of this in a way that is non-directive, unhurried, and free of any pressure toward a particular reproductive decision. This is a HOTS objective precisely because the knowledge (the tests, the numbers) is the easy part — the higher-order skill is weaving probability, choice, and emotionally loaded information into a conversation the woman controls.
The clinical pathway is a funnel. Every woman is offered (never required) a risk-assessment screen. Those with a higher-chance result are offered diagnostic testing, which is the only way to confirm or exclude the diagnosis. At each gate the woman may decline and step off. The counsellor's job is to keep the gates open and the woman in the driving seat, not to push her through. In South Africa this happens across a steep resource gradient: a first-trimester combined screen needs an accredited nuchal-translucency sonographer and a biochemistry assay that many district facilities do not have, so the realistic offer at primary level is often maternal-age risk plus a second-trimester anomaly scan, with referral upward for formal screening and any diagnostic procedure. Honest counselling names what is actually available locally rather than reciting an idealised menu.
Core knowledge
Cytogenetics and recurrence
Down syndrome results from three mechanisms, and the distinction matters for recurrence counselling:
- Free trisomy 21 (~94%) — meiotic non-disjunction, usually maternal. Strongly age-related. Recurrence after an affected pregnancy is modestly raised above the age-specific background (standard teaching is roughly an extra ~0.5–1% above the woman's age risk, though figures vary with the source — quote it cautiously).
- Robertsonian translocation (~4%) — e.g. der(14;21) or der(21;21). Here the parents must be karyotyped, because a balanced carrier parent carries a high, age-independent recurrence risk. A parent carrying an isochromosome 21q [t(21;21)] has effectively a 100% recurrence risk and warrants explicit discussion of donor gametes or pre-implantation genetic testing.
- Mosaicism (~2%) — post-zygotic non-disjunction; phenotype is variable and may be milder.
The age relationship is the single most counselled fact: maternal-age risk of trisomy 21 rises slowly through the twenties and steeply after the mid-thirties. Standard teaching quotes the term risk at age 35 as roughly 1 in 350 and at age 40 as roughly 1 in 100, but the exact figures differ between reference tables and between gestations (the live-birth risk is lower than the first-trimester risk because of fetal loss), so present them as orders of magnitude, not precise quotes.
Figure K2.1 — Cytogenetic mechanisms of Down syndrome and how each changes recurrence counselling, including maternal-age risk anchors.
What screening actually estimates
Screening does not diagnose. It converts a woman's prior risk (chiefly her age) into a posterior, individualised chance by combining markers. The combined first-trimester screen pairs ultrasound nuchal translucency (NT) with two maternal serum analytes — free β-hCG (raised in affected pregnancies) and PAPP-A (lowered) — performed in the 11+0 to 13+6 week window where a crown– rump length of roughly 45–84 mm allows reliable NT measurement. The result is a ratio (e.g. "1 in 1500" = lower chance; "1 in 50" = higher chance) against a chosen cut-off. The combined test detects in the order of ~85–90% of affected pregnancies for a ~5% screen-positive (false-positive) rate — quote these as approximate performance figures, as they vary by laboratory and cohort.
Cell-free DNA (cfDNA / NIPT) analyses placentally derived fragments in maternal plasma from ~10 weeks. For trisomy 21 it has the highest detection (often quoted
99%) with a very low false-positive rate — but it remains a screening test, not diagnostic, because the DNA is placental (confined placental mosaicism, vanishing twin, and rarely maternal copy-number variation or malignancy cause discordance). A "high-chance" cfDNA result must still be confirmed by an invasive diagnostic test before any irreversible decision. cfDNA can fail to return a result ("no-call"), itself associated with a higher aneuploidy risk, and performs less reliably at low fetal fraction (e.g. early gestation, higher maternal weight).
Diagnostic tests
Only chorionic villus sampling (CVS) and amniocentesis give a fetal karyotype (or rapid result by QF-PCR/FISH then full array/karyotype):
- CVS — transabdominal or transcervical placental biopsy, from ~11 weeks. Earlier result, but carries the small caveat of confined placental mosaicism. Not performed before ~10–11 weeks because of an association with limb-reduction defects when done too early.
- Amniocentesis — transabdominal aspiration of amniotic fluid, from ~15 weeks (early amniocentesis before 15 weeks is avoided — higher loss and talipes).
Both carry a small procedure-related miscarriage risk. Contemporary estimates in experienced hands are lower than the historical "1%" teaching — often quoted around ~0.1–0.2% above background for amniocentesis — but the figure is operator- and volume-dependent, so counsel the local unit's audited rate where known and present the number honestly rather than reciting a textbook constant.
Assessment
The counselling encounter
Pre-test counselling is itself the assessment. Establish, in unhurried language and the woman's own tongue (with a trained interpreter, never a family member, where needed):
- What she already knows and wants to know. Some women want every number; others want only "is the baby healthy?". Calibrate to her.
- The purpose of screening — that it gives a chance, not a yes/no, and that a "higher-chance" result does not mean the baby is affected.
- What she would do with the information. This is explored, not directed. Some decline screening because the result would not change their pregnancy; that is a valid, autonomous choice and must be respected and documented.
- The downstream pathway — that a higher-chance screen leads to an offer of diagnostic testing, which carries a small procedure risk, and that she may stop at any point.
Confirming gestation and eligibility
Accurate dating underpins everything: serum analyte medians and NT cut-offs are gestation-specific, so a reliable crown–rump length (or, later, biometry) is essential before a screen can be interpreted. Confirm singleton versus multiple (screening performance and analyte interpretation differ in twins; cfDNA is less established and serum screening less accurate in multiples). Document maternal weight, smoking, diabetes, IVF and chorionicity, as these adjust analyte medians.
Interpreting and delivering a result
A screen result is delivered as a chance against the cut-off, framed in absolute and natural-frequency terms ("about 1 in 80 — so of 80 women with this result, around one will have an affected baby and 79 will not"). Avoid the loaded shorthand "positive/ negative"; use "higher chance / lower chance". Where soft markers are seen on the anomaly scan (e.g. nuchal fold, echogenic bowel, short femur, absent nasal bone), these are modifiers of risk, not diagnoses, and are best interpreted in a fetal-medicine setting against the combined/cfDNA result rather than acted on in isolation.
Management
Stepwise pathway
- Booking offer. At first antenatal contact, offer screening to every woman regardless of age, with balanced information and the explicit option to decline. Record the choice.
- First-trimester combined screen (11+0–13+6 wk) where NT sonography and serum biochemistry are available. Where they are not (much of SA primary care), the realistic offer is age-based risk plus a second-trimester structural scan, with referral upward for formal screening.
- cfDNA — used as a primary screen where affordable, or (cost-effectively) as a contingent second-line test for those with a higher-chance combined result, which reduces the number of women proceeding to invasive testing.
- Higher-chance result → offer diagnostic test. Non-directive counselling on CVS versus amniocentesis, their timing and procedure risks. The woman chooses; some decline and continue without confirmation.
- Diagnostic result. A confirmed diagnosis triggers a further, separate counselling episode — ideally with fetal medicine, genetics, neonatology and parent-support input — covering the spectrum of the condition (intellectual disability is variable; associated cardiac, gastrointestinal, haematological and thyroid issues), and the woman's lawful options.
Figure K2.2 — Stepwise Down syndrome screening pathway from booking offer to diagnostic confirmation, emphasizing that screening gives a chance rather than a diagnosis.
South African context and the law
- Availability. Accredited NT sonography and PAPP-A/free-βhCG assays are concentrated in tertiary and private settings; cfDNA is largely self-funded and out of reach for most public-sector women. Counselling must be truthful about what is actually offered at that facility and about realistic referral, not an idealised European pathway. Maternal-age-based risk discussion and the second-trimester anomaly scan remain the universally available backbone.
- Referral. Higher-chance results and all invasive procedures are referred to a regional/tertiary unit with fetal-medicine capacity. This aligns with the level-of-care structure in the SA National Integrated Maternal and Perinatal Care Guideline.
- The law on termination. Reproductive options after a confirmed diagnosis are governed by the Choice on Termination of Pregnancy Act 92 of 1996 (as amended). Counselling around lawful options must be non-directive and consistent with HPCSA ethical guidance and the informed-consent requirements of the National Health Act 61 of 2003 — the woman decides; the clinician informs and supports. Continuing the pregnancy is an equally supported choice, and the counselling must signal that explicitly so that no woman feels steered.
Consent and documentation
Screening and diagnostic testing are interventions requiring valid informed consent: the woman must understand the nature, purpose, alternatives (including declining) and material risks. For an invasive test, the procedure-related loss risk is a material risk that must be disclosed. Document the offer, the information given, her questions, and her decision at each gate.
Red flags / pitfalls
- Treating a screen as a diagnosis. The commonest and most damaging error. Never let "higher chance" be heard as "your baby has Down syndrome", and never act irreversibly (including termination) on a screen — including a high-chance cfDNA — without diagnostic confirmation. cfDNA is screening, not diagnostic.
- Directive counselling. Steering a woman toward testing or toward termination (or away from it) breaches autonomy and HPCSA ethics. The lawful, ethical default is non-directive: lay out options, support whatever she chooses, document that continuing is equally supported.
- Offering only by age. Screening is offered to all women, not just those over 35; most affected babies are born to younger women simply because younger women have far more pregnancies. Age-only triage misses the majority.
- Wrong dates, wrong result. Interpreting analytes or NT against an inaccurate gestation produces a meaningless risk. Confirm dating first.
- Forgetting parental karyotypes in translocation Down syndrome. A translocation result mandates parental karyotyping for recurrence counselling — missing a balanced carrier parent (especially t(21;21)) is a serious recurrence-counselling failure.
- Quoting false precision. Procedure-loss rates and age risks vary by source and operator. Present ranges and the local audited figure where known; never invent a precise number to sound authoritative.
- Skipping the interpreter or rushing. Emotionally and numerically dense counselling delivered through a relative or in a language barrier is not informed consent. Use a trained interpreter and allow time.
Counselling-integrity drill — a higher-chance result is NOT a diagnosis. When a screen returns higher-chance (combined OR cfDNA): (1) state explicitly it is a chance, in natural frequency; (2) confirm dating and result validity; (3) offer — never impose — a diagnostic test (CVS or amniocentesis) as the ONLY way to confirm; (4) take NO irreversible step on screening alone; (5) counsel non-directively, documenting that continuing the pregnancy is equally supported; (6) refer to fetal medicine/genetics. A reversible pathway protected by diagnostic confirmation is the safety net — never bypass it.
Figure K2.3 — Counselling safety-net for a higher-chance screen result, preserving autonomy, diagnostic confirmation and South African ethical/legal context.
Evidence anchors
- Aneuploidy screening principles — combined first-trimester screen (NT + free β-hCG + PAPP-A, 11+0–13+6 wk), cell-free DNA / NIPT, and diagnostic CVS/ amniocentesis are the standard pathway (per the genetics screening framework in
docs/VERIFIED-SOURCES.md); see also principles-of-inheritance for the cytogenetic basis (non-disjunction, Robertsonian translocation, mosaicism). - South African National Integrated Maternal and Perinatal Care Guideline, 5th edition (2024) — antenatal booking, the level-of-care/referral structure, and routine second-trimester ultrasound that underpin the realistic SA screening offer.
- NICE NG201 — Antenatal care (2021) — informs the framework for offering aneuploidy screening at booking with balanced, non-directive information and the option to decline.
- Choice on Termination of Pregnancy Act 92 of 1996 (as amended) — governs lawful reproductive options after a confirmed diagnosis.
- HPCSA ethical guidelines and the National Health Act 61 of 2003 — informed consent and the duty of non-directive counselling; see informed-consent and sa-og-law.
- Author's note on hedged facts: the specific detection/false-positive rates (~85–90% / ~5% for combined; >99% for cfDNA), maternal-age risk figures (~1 in 350 at 35, ~1 in 100 at 40), recurrence increments, and procedure-related loss rates (~0.1–0.2% for amniocentesis) are standard teaching presented cautiously as approximate ranges — they are NOT line-itemed with exact values in
docs/VERIFIED-SOURCES.mdand vary by laboratory, cohort and operator, so they are deliberately hedged rather than cited to a precise source.