SBA — Single Best Answer · Review
Question 26
Feto-maternal medicine → Perinatology
Classification
Question
SSBAQuestion Header
Third-trimester amniocentesis
Question Stem
A woman undergoes amniocentesis late in the third trimester for a specific fetal diagnostic indication. Which laboratory problem becomes increasingly important as gestation advances?
Options
A
Universal maternal cell contamination
B
Universal rapid-test failure
C
Confined placental mosaicism
D
Failure to obtain any amniotic fluid
E
Increasing culture failure
Explanation
Evidence for third-trimester amniocentesis is limited, but culture failure becomes more frequent with advancing gestation. RCOG cites an overall culture-failure rate of approximately 10% in one large dataset, with rates increasing substantially at later gestations. Late procedures may also have higher rates of blood-stained samples and the need for more than one needle insertion. These limitations should form part of counselling when late invasive diagnosis is considered.
Option Validity
A) Maternal cell contamination can occur but is not universal and is not the principal gestation-dependent laboratory issue described.
B) Rapid-test failure is not described as universal in late gestation.
C) Confined placental mosaicism is principally a CVS issue because chorionic villi are placental tissue.
D) Late amniocentesis does not universally fail to obtain fluid; the important problem is increasing culture failure.
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.