PU Multiple Gestation Ultrasound 1 — Questions and Answers
Question 1: Which ultrasound finding best distinguishes dichorionic-diamniotic (DCDA) twins from monochorionic-diamniotic (MCDA) twins in the first trimester?
- Number of yolk sacs visible
- Twin peak (lambda) sign at placental insertion (Correct answer)
- Presence of any dividing membrane
- Crown-rump length discordance greater than 5 days
Correct answer: Twin peak (lambda) sign at placental insertion
The lambda (twin peak) sign represents chorionic villi extending into the base of the inter-twin membrane, indicating dichorionic placentation.
Question 2: The 'T-sign' on ultrasound at the placental insertion of the inter-twin membrane is associated with which type of twin placentation?
- Dichorionic-diamniotic
- Monochorionic-diamniotic (Correct answer)
- Monochorionic-monoamniotic
- Dichorionic-monoamniotic
Correct answer: Monochorionic-diamniotic
The T-sign occurs in monochorionic-diamniotic twins because the thin membrane inserts perpendicularly into the placenta without chorionic villi filling the base.
Question 3: Twin-to-twin transfusion syndrome (TTTS) occurs exclusively in which type of twin pregnancy?
- Dichorionic-diamniotic
- Monochorionic-diamniotic (Correct answer)
- Monochorionic-monoamniotic
- Any dizygotic twin pregnancy
Correct answer: Monochorionic-diamniotic
TTTS requires vascular anastomoses within a shared monochorionic placenta, and diamniotic twins are the most common form in which it is diagnosed and treated.
Question 4: According to the Quintero staging system for TTTS, which finding defines Stage I?
- Absent end-diastolic flow in the donor's umbilical artery
- Hydrops in the recipient twin
- Oligohydramnios in donor and polyhydramnios in recipient with a still-visible donor bladder (Correct answer)
- Fetal demise of one twin
Correct answer: Oligohydramnios in donor and polyhydramnios in recipient with a still-visible donor bladder
Quintero Stage I is defined by the oligohydramnios-polyhydramnios sequence (TOPS) with the donor's bladder still visible on ultrasound.
Question 5: At what gestational age can chorionicity be most reliably determined by ultrasound?
- 6–7 weeks
- 11–14 weeks (Correct answer)
- 16–18 weeks
- 20–24 weeks anatomy scan
Correct answer: 11–14 weeks
The lambda and T-signs are most reliably identified between 11 and 14 weeks, making this the optimal window for chorionicity determination.
Question 6: Which ultrasound finding is most characteristic and pathognomonic of monochorionic-monoamniotic (MCMA) twins?
- Lambda sign at placental insertion
- Thin dividing membrane measuring less than 2 mm
- Umbilical cord entanglement between the twins (Correct answer)
- Significant estimated fetal weight discordance
Correct answer: Umbilical cord entanglement between the twins
Cord entanglement is pathognomonic for MCMA twins because both fetuses share a single amniotic cavity with no membrane separating their cords.
Question 7: In twin pregnancies, what percentage of estimated fetal weight (EFW) discordance between twins is generally considered clinically significant?
- Greater than 5%
- Greater than 10%
- Greater than 20% (Correct answer)
- Greater than 30%
Correct answer: Greater than 20%
An EFW discordance exceeding 20% between twins is the widely accepted threshold for clinically significant growth discordance requiring closer surveillance.
PU Multiple Gestation Ultrasound 1
Which ultrasound finding best distinguishes dichorionic-diamniotic (DCDA) twins from monochorionic-diamniotic (MCDA) twins in the first trimester?