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Question 21
Feto-maternal medicine → Perinatology
Classification
Question
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Confined placental mosaicism
Question Stem
Chorionic villus sampling is performed following an abnormal screening result. There is no fetal structural abnormality, but rapid QF-PCR testing suggests a chromosomal abnormality. What is the most appropriate next step before making an irreversible pregnancy management decision?
Options
A
Repeat QF-PCR on the same sample only
B
Await the full karyotype result
C
Proceed immediately to termination on the rapid result alone
D
Repeat maternal serum screening
E
Ignore the result because CVS cannot diagnose aneuploidy
Explanation
Confined placental mosaicism occurs in approximately 1–2% of chorionic villus samples. When there is no corresponding fetal structural abnormality and rapid testing suggests a chromosomal abnormality, RCOG advises caution before an irreversible decision because the placental result may not represent the fetal karyotype. The full cytogenetic result should therefore be awaited and further fetal testing considered where appropriate.
Option Validity
A) Repeating the same rapid assay does not resolve the issue of confined placental mosaicism.
C) An irreversible decision should not be based solely on the rapid placental result in this situation.
D) Screening does not supersede definitive cytogenetic assessment after an abnormal invasive diagnostic result.
E) CVS is a definitive diagnostic procedure, but placental mosaicism can complicate interpretation.
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.