EMQ — Extended Matching Question · Review
Question 31
Feto-maternal medicine → Perinatology
Classification
Question
EEMQAnswer Bank
Correct Answer
Explanation
Chorionic villus sampling examines placental tissue and can therefore be affected by confined placental mosaicism. RCOG notes that confined placental mosaicism occurs in approximately 1-2% of CVS samples. Where rapid testing such as QF-PCR suggests a chromosomal abnormality but there is no corresponding structural fetal abnormality, the full karyotype should be awaited before an irreversible pregnancy decision is made. This reduces the risk of acting on a result that reflects the placenta rather than the fetus. Immediate repeat CVS or unrelated imaging does not resolve the underlying issue as appropriately as confirmatory cytogenetic assessment.
Option Validity
A) A rapid result alone may be misleading because of confined placental mosaicism.
C) Immediate repeat CVS samples placental tissue again and is not the recommended first response.
D) Routine delay until 20 weeks is not the recommended confirmatory strategy.
E) Maternal blood testing does not provide the required confirmation of the fetal karyotype.
F) CVS is a definitive diagnostic procedure but has recognised limitations including confined placental mosaicism.
G) Fetal MRI does not confirm the chromosomal result.
H) Ultrasound surveillance alone does not provide cytogenetic confirmation.
Further Reading
Navaratnam K, Alfirevic Z; Royal College of Obstetricians and Gynaecologists. Amniocentesis and chorionic villus sampling: Green-top Guideline No. 8. BJOG 2022;129:e1-e15. doi:10.1111/1471-0528.16821.