In one line
Aneuploidy screening estimates a probability, never a diagnosis: every screen-positive woman is offered confirmatory invasive testing before any irreversible decision. The positive predictive value of even cell-free DNA rises and falls with the woman's prior risk, so the same "positive" result means very different things in a 40-year-old and a 22-year-old — which is why no single PPV figure can be quoted for it.
Mechanism & pathophysiology
Aneuploidy is a deviation from the euploid 46-chromosome complement, almost always arising from meiotic non-disjunction — failure of homologues (meiosis I) or sister chromatids (meiosis II) to separate, yielding a gamete with 24 or 22 chromosomes. The maternal oocyte, arrested in meiosis I from fetal life until ovulation decades later, accumulates cohesin degradation over time; this is the mechanistic basis of the steep, non-linear rise in trisomy with maternal age, and it is why age alone is the oldest and crudest screening filter.
The clinically relevant aneuploidies behave according to which chromosome carries the extra (or missing) material:
- Trisomy 21 (Down syndrome) — the commonest viable autosomal trisomy (~1 in 700 livebirths overall, far higher with advancing age). Chromosome 21 is gene-poor, which is why the trisomy is survivable; the screening markers it produces are the reference against which every test is calibrated.
- Trisomy 18 (Edwards) and trisomy 13 (Patau) — gene-richer chromosomes, hence overwhelmingly lethal in utero or in early infancy. Their biochemical signature differs from T21 (notably low PAPP-A and low free β-hCG in T18, distinct from the high β-hCG of T21), and they frequently carry major structural anomalies visible on ultrasound, so they are often suspected on the anomaly scan independent of the serum screen.
- Monosomy X (Turner syndrome, 45,X) — usually paternal sex-chromosome loss; most are lost early, the survivors typically presenting with cystic hygroma, hydrops or a raised nuchal translucency rather than a serum-marker pattern.
- Triploidy (69,XXX / 69,XXY) — a whole extra haploid set, digynic (maternal, small placenta, severe early growth restriction) or diandric (paternal, partial molar placenta with markedly raised β-hCG). It is non-viable, and recognising its biochemistry prevents misclassification.
The serum and sonographic markers exploited by screening are downstream consequences of abnormal placentation and fetal development. Nuchal translucency (NT) — the fluid collection at the fetal neck measured at 11–13⁺⁶ weeks — is increased in T21, T18, T13, Turner syndrome and in structurally normal fetuses with cardiac defects, reflecting transient lymphatic/cardiac dysfunction. PAPP-A (pregnancy-associated plasma protein-A) and free β-hCG are placental products: in T21 PAPP-A is low and free β-hCG high; in the trisomies 18 and 13 both tend to be low. The second-trimester quadruple markers — AFP (fetal-origin, low in T21), hCG (high in T21), unconjugated oestriol (uE3) (low in T21) and inhibin A (high in T21) — capture the same disordered feto-placental endocrinology later in gestation.
Cell-free DNA (cfDNA) is a different mechanism entirely, and its name is a half-truth that the counselling must respect. From around 4 weeks, fragments of DNA circulate in maternal plasma. The fraction that is "fetal" is in fact placental — it derives from apoptotic cytotrophoblast, not the fetus directly. That single fact explains every important limitation of the test:
- The proportion of the total cfDNA that is placental is the fetal fraction. Below roughly 4% the assay cannot reliably call a result and returns a "no-call"; fetal fraction falls with high maternal weight (dilution by maternal cfDNA), early gestation and certain aneuploidies (notably T18, T13 and triploidy, where the placenta is small), so a no-call is itself a soft risk signal, not merely a technical nuisance.
- Because the DNA is trophoblastic, a placenta that is genetically discordant from the fetus — confined placental mosaicism — produces a false-positive (or, rarely, false-negative) cfDNA result that the actual fetal karyotype does not share. This is the dominant biological reason cfDNA is a screen, not a diagnosis.
- Other sources of placental or maternal DNA confound the result: a vanishing twin continues to shed trophoblastic DNA for weeks; maternal copy-number variants, mosaicism or an occult malignancy contribute aneuploid-appearing maternal DNA and are a recognised, if rare, cause of an uninterpretable or falsely positive result that occasionally unmasks maternal disease.
Two patterns are measured: counting-based (massively parallel shotgun sequencing or targeted counting of chromosome-specific reads against a reference) and SNP-based methods. The performance figures below are broadly method-independent for the common trisomies.
Assessment
Screening is a structured pathway, not a single test, and the assessment is as much about who the woman is as about the marker values.
- Establish dates and chorionicity first. Every risk algorithm is gestation-dependent (NT and PAPP-A/β-hCG have narrow valid windows); a misdated pregnancy generates a spurious risk. In multiple pregnancy, chorionicity changes both the validity and the interpretation of every method.
- Quantify the prior risk — maternal age, gestation, previous affected pregnancy, parental balanced translocation. This baseline is what the screen modifies; it is also what determines the post-test predictive value of any positive result, and it must be elicited explicitly rather than assumed.
- First-trimester combined test (11–13⁺⁶ weeks): maternal age + NT + PAPP-A + free β-hCG. Reported detection for T21 is of the order of 85–90% at a ~5% false-positive rate. Its advantages are early timing (earlier reassurance or earlier decision), an NT that also flags non-aneuploid structural disease, and the chance to identify T18/T13 by their distinct biochemistry.
- Second-trimester quadruple test (≈14–20 weeks): maternal age + AFP + hCG + uE3 + inhibin A, for women who book late or where first-trimester NT is unavailable — common in the SA public sector. Detection for T21 is lower (~75–80% at a 5% FPR). The quadruple test screens for T21 (and AFP flags open neural-tube defects) but is not designed to give a discrete T18/T13 risk in the way the combined test does.
- Integrated and contingent strategies combine first- and second-trimester information to lift detection and cut the false-positive rate, at the cost of delaying the result. The contingent model — perform a cheap first-line test on everyone, reserve the expensive second-line test (cfDNA) for those whose first-line risk crosses a threshold — is the structure that makes high-performance screening affordable in a constrained system.
- cfDNA / NIPT can be deployed as a primary screen offered to all, or as a contingent second-line test after a high-risk combined or quadruple result. The interpretation hinges on prior risk (below). A no-call result warrants active counselling with an offer of ultrasound and diagnostic testing (a redraw is an option, not a prerequisite), not silent re-bleeding.
Interpreting a result means converting a risk into a plan: a low-risk screen reduces but never abolishes risk (the residual is the false-negative rate); a high-risk screen is an invitation to confirmatory testing, not a diagnosis. The number a woman is told — "1 in 80", "high-chance" — is meaningless to her unless the counselling makes clear what proportion of women with that result actually carry an affected fetus.
Screening in multiple pregnancy and the limits of soft markers
Two assessment situations trip up an otherwise sound plan: twins and the incidental "soft marker".
In multiple pregnancy every method degrades. Serum biochemistry averages two feto-placental units, so a normal twin can mask an affected co-twin and blunt detection. NT is the more useful first-trimester tool because it is fetus-specific and lets each twin carry its own risk, and it doubles as the chorionicity and discordance assessment that governs the whole pregnancy. cfDNA performs respectably for T21 in twins but with a higher no-call rate (the fetal fraction is split, and a low contribution from one placenta can drop the assay below threshold), and it cannot localise an abnormal signal to one twin — so a positive cfDNA in twins still leads to fetus-specific invasive testing. A vanishing twin is a specific cfDNA trap: the demised co-twin's placenta sheds discordant trophoblastic DNA for weeks, producing a false-positive that an early dating scan documenting the loss should pre-empt.