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Question 38
Feto-maternal medicine → Multiple pregnancy
Classification
Question
SSBAQuestion Header
Chorionic villus sampling in twin pregnancy
Question Stem
A dichorionic twin pregnancy requires chorionic villus sampling of both placentas. The woman is counselled that cross-contamination between samples can occur. Which procedural approach may reduce this risk?
Options
A
Sampling both placentas through the same needle track whenever possible
B
Performing the procedure before 10+0 weeks
C
Combining both placental samples before laboratory analysis
D
Avoiding ultrasound guidance during the second sample
E
Using a separate uterine entry for each placental sample
Explanation
In twin pregnancies undergoing chorionic villus sampling, cross-contamination between samples is a recognised problem and is reported at approximately 1%. RCOG notes that the risk may be reduced by using a double uterine entry technique, allowing the placental samples to be obtained separately. Accurate fetal and placental mapping and clear sample identification are also important in multiple pregnancy, particularly where subsequent selective intervention might be considered.
Option Validity
A) A single shared sampling route does not represent the strategy described by RCOG for reducing cross-contamination between twin CVS samples.
B) CVS should not be performed before 10+0 weeks and, where possible, should be performed from 11+0 weeks.
C) Combining the samples would prevent reliable attribution of the diagnostic result to the appropriate fetus.
D) Invasive prenatal procedures should be performed under continuous ultrasound guidance.
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.