O&G SAQs · Antenatal care — prenatal diagnosis
Prenatal diagnosis — structured SAQ (15 marks)
FRANZCOG-format structured SAQ on prenatal diagnosis: CVS versus amniocentesis, the modern loss rate, counselling a trisomy 21 result, and the microarray-over-karyotype strategy for a structural anomaly with the Wapner incremental yield. Per-sub-part marking rubric included.
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Prompt
A 37-year-old woman at 11 weeks' gestation has a cell-free DNA screen reporting a 1 in 15 risk of trisomy 21. She wants a definitive answer. (a) Compare chorionic villus sampling and amniocentesis for this woman: timing, tissue sampled, and one limitation of each. (5 marks) (b) She chooses CVS. State the modern procedure-related fetal loss risk you will quote and the source. (2 marks) (c) The result returns trisomy 21. Outline your counselling and the options you would offer. (4 marks) (d) In a separate patient, a fetal cardiac anomaly is found at 18 weeks with a normal karyotype. Explain your laboratory strategy and the expected incremental yield. (4 marks)
How this SAQ is marked
Twelve SAQs, 180 marks, two 2-hour papers — roughly 15 marks and 20 minutes each. Marks come from specifics: gestation windows, tissue sampled, the modern loss rate, and the platform choice. Write in short labelled points. [1]
Reveal model answer and mark schemeShowHide
(a) CVS versus amniocentesis (5 marks)
One mark per comparison point, maximum five. [1]
- CVS timing: 10 to 13 weeks (an earlier definitive answer at 11 weeks suits this woman).[1]
- CVS tissue: placental chorionic villi (trophoblast); transabdominal or transcervical.[1]
- CVS limitation: rare confined placental mosaicism — a mosaic result may need amniocentesis to confirm the fetus.[1]
- Amniocentesis timing: 15 to 20 weeks (later answer); samples fetal cells in amniotic fluid.[1]
- Amniocentesis limitation: not available at 11 weeks; offers AFP and acetylcholinesterase for neural tube defects.[1]
(b) Modern procedure-related loss risk (2 marks)
One mark for the figure, one for the source. [2]
- The Salomon 2019 systematic review and meta-analysis estimated a weighted procedure-related miscarriage risk of about 0.20% for CVS and 0.30% for amniocentesis above background, close to zero in matched-risk studies.[2]
- The historic 1% figure has been superseded; quote the modern number.[1][2]
(c) Counselling a trisomy 21 result (4 marks)
One mark per point, maximum four. [1]
- Break the result humanly, confirm understanding, involve clinical genetics.[1]
- Counsel on what trisomy 21 means for this pregnancy — phenotype and prognosis — checking understanding as you go.[1]
- Lay out the options non-directively: continue the pregnancy with preparation and support; termination of pregnancy within the legal framework.[1]
- Offer ongoing fetal-medicine and paediatric follow-up, and a written summary with a named contact.[1]
(d) Laboratory strategy for a cardiac anomaly with normal karyotype (4 marks)
One mark per point, maximum four. [3][4]
- First-tier test: chromosomal microarray (not repeat karyotype) — detects submicroscopic copy-number variants the karyotype misses.[3][4]
- Expected yield: clinically relevant copy-number variants in about 6.0% of structurally abnormal fetuses with a normal karyotype (Wapner 2012).[3]
- Blind spot: microarray does not detect balanced translocations or triploidy; if either is suspected, a karyotype is also needed.[3]
- Next step if microarray normal and a single-gene disorder is suspected: refer to clinical genetics to discuss further testing options.[1]
References4ShowHide
- [1]Royal College of Obstetricians and Gynaecologists (RCOG) Amniocentesis and Chorionic Villus Sampling: Green-top Guideline No. 8 BJOG, 2010.Source
- [2]Salomon LJ, Sotiriadis A, Wulff CB, et al. Risk of miscarriage following amniocentesis or chorionic villus sampling: systematic review of literature and updated meta-analysis Ultrasound Obstet Gynecol, 2019.PMID 31124209
- [3]Wapner RJ, Martin CL, Levy B, et al. Chromosomal microarray versus karyotyping for prenatal diagnosis N Engl J Med, 2012.PMID 23215555
- [4]Miller DT, Adam MP, Aradhya S, et al. Consensus statement: chromosomal microarray is a first-tier clinical diagnostic test for individuals with developmental disabilities or congenital anomalies Am J Hum Genet, 2010.PMID 20466091