Prenatal screening for chromosomal conditions — structured oral station (12 minutes)

Opening prompt — "Interpret her result for me."

Model response — lead with the classification, then the number that makes it: [3]

• "Her combined screening risk for trisomy 21 is 1 in 150, which is at the screen-positive threshold — typically 1 in 300 or 1 in 150 depending on the laboratory. So this is a screen-positive result." [3]
• "It is a probability, not a diagnosis. Most women with a screen-positive result have an unaffected fetus — the result tells us she is at higher risk than baseline, and the next step is a diagnostic test."

Probe 1 — "What is the performance of the screen she had?"

• "First-trimester combined screening, performed at 11 weeks, detects about 87 percent of trisomy-21 fetuses at a 5 percent false-positive rate — that is from the Malone FIRST trial in the New England Journal. Performance falls a little at 12 and 13 weeks."[2]
• "The combined screen joins nuchal translucency, PAPP-A, free beta-hCG and maternal age. Her raised NT is driving the risk."

Probe 2 — "She asks about cfDNA. How does it compare?"

This is the discriminating probe. Quote both performance pairs. [1][2]

• "cfDNA — the blood test looking at placental DNA fragments — has better performance. The Gil meta-analysis found a detection rate of 99.7 percent and a false-positive rate of 0.04 percent for trisomy 21, so roughly 99 percent detection at under 0.1 percent false-positive."[1]
• "That means cfDNA detects more and false-positives far fewer. The trade-off is cost, the possibility of a no-call result if the fetal fraction is low, and that it screens for a narrower range of conditions than a combined programme plus the anomaly scan."[1]

Probe 3 — "She says she will 'just get the cfDNA and act on it'. Is that safe?"

• "No, and I would counsel her carefully. cfDNA is a screening test, not a diagnostic one. It samples placental DNA, not fetal cells directly — so if the placenta carries a chromosomal abnormality the fetus does not, a condition called confined placental mosaicism, the cfDNA is positive but the fetus is normal. A proportion of cfDNA positives are false positives for that reason."[1]
• "So I would tell her: cfDNA is excellent at stratifying risk, but any positive — from combined screening or cfDNA — is confirmed by a diagnostic test before any irrevocable decision."[3]

Probe 4 — "What is the diagnostic test, and what is the risk?"

• "At 12 weeks the diagnostic test is chorionic villus sampling — sampling placental trophoblast, transabdominally, with rapid QF-PCR and a full karyotype or microarray. Amniocentesis is the alternative from 15 weeks."[3]
• "The procedural risk is much lower than many women fear. Contemporary data from Akolekar show the miscarriage rate after CVS was 1.5 percent versus 1.2 percent in untested controls — not statistically significant — and after amniocentesis 0.8 percent versus 1.2 percent. The procedure-related risk with modern ultrasound-guided sampling is close to zero."[4]
• "I would put that in plain terms: the test is safe, and it gives her certainty."

Probe 5 — "What if the cfDNA comes back as a no-call?"

• "A no-call means the test failed because the fetal fraction was too low — it is not a negative result. Low fetal fraction is associated with a higher background rate of aneuploidy and adverse outcome."[5]
• "So I would not reassure her. The options are a repeat cfDNA — which may also fail, particularly if her BMI is raised — a switch to combined screening if she is still in the window, or offering diagnostic testing. We resolve the risk; we do not file a no-call as normal."[5]

Probe 6 — communication: "Explain the plan to her in plain language."

Three of the eight domains are scored here. Say the words out loud. [3]

• "Your screening test puts you at higher risk, but most women in your position have a healthy baby. The screening test tells us the chance, not the answer." [3]
• "The way to get the answer is a small test called CVS, where we take a tiny sample from the placenta and look at the baby's chromosomes. It is done through the tummy with local anaesthetic, it takes a few minutes, and the miscarriage risk is very low — close to the background rate."
• "If the result is normal, you can be reassured. If it shows trisomy 21, we will sit down and go through what that means, and you will have time to decide what you want to do. Nothing happens in a rush."
• Check understanding by asking her to say the plan back, and offer written information and a follow-up call.