SBA — Single Best Answer · Review
Question 23
Feto-maternal medicine → Multiple pregnancy
Classification
Question
SSBAQuestion Header
Invasive diagnosis in twins
Question Stem
A dichorionic twin pregnancy requires invasive prenatal diagnosis. Which method is most appropriate for identifying the fetuses so that subsequent results can be reliably attributed to the correct twin?
Options
A
Label the fetuses twin 1 and twin 2 only
B
Identify each fetus according to estimated fetal weight
C
Identify each fetus according to fetal sex
D
Map each fetus using fixed maternal anatomical orientation such as maternal left/right or upper/lower
E
Use presentation alone because this remains constant throughout pregnancy
Explanation
Before invasive testing in a multiple pregnancy, detailed pregnancy mapping is essential so that laboratory results can be attributed to the correct fetus. RCOG recommends using fixed maternal anatomical orientation, such as maternal left/right or upper/lower, rather than simply assigning numbers. Where distinguishing features are absent, two operators should confirm the fetal labelling.
Option Validity
A) Numbering alone may lead to confusion and is less reliable than fixed anatomical mapping.
B) Estimated fetal weight can change relative to the co-twin and is not a reliable identifier.
C) Fetal sex may be the same and is not an adequate universal mapping method.
E) Fetal presentation can change during pregnancy and should not be used as the principal identifier.
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.