PU Multiple Gestation Ultrasound 2 — Questions and Answers
Question 1: What defines Quintero Stage II in twin-to-twin transfusion syndrome (TTTS)?
- Oligohydramnios-polyhydramnios sequence with an absent donor bladder (Correct answer)
- Absent or reversed end-diastolic flow in the umbilical artery
- Hydrops in the recipient twin
- Fetal demise of one or both twins
Correct answer: Oligohydramnios-polyhydramnios sequence with an absent donor bladder
Stage II is characterized by the TOPS sequence progressing to the point where the donor's bladder is no longer visible due to oliguria from severe volume depletion.
Question 2: Selective intrauterine growth restriction (sIUGR) in monochorionic twins is primarily defined by which finding?
- EFW discordance >20% with normal umbilical artery Dopplers
- EFW of one twin below the 10th percentile for gestational age (Correct answer)
- EFW discordance >10% with abnormal Dopplers in the smaller twin
- Polyhydramnios developing in the larger twin's sac
Correct answer: EFW of one twin below the 10th percentile for gestational age
sIUGR is defined as EFW below the 10th percentile in one monochorionic twin, distinguishing it from TTTS which requires the oligohydramnios-polyhydramnios sequence.
Question 3: Twin anemia-polycythemia sequence (TAPS) is best identified on ultrasound by which Doppler finding?
- Absent end-diastolic flow in the donor twin's umbilical artery
- Middle cerebral artery peak systolic velocity (MCA-PSV) >1.5 MoM in the anemic twin (Correct answer)
- Reversed a-wave in the ductus venosus of the polycythemic twin
- Elevated umbilical vein pulsatility index
Correct answer: Middle cerebral artery peak systolic velocity (MCA-PSV) >1.5 MoM in the anemic twin
MCA-PSV >1.5 MoM is the standard Doppler marker for fetal anemia; in TAPS the donor becomes anemic, and this finding distinguishes TAPS from TTTS.
Question 4: In a naturally conceived triplet pregnancy, what is the most common chorionicity-amnionicity configuration?
- Trichorionic-triamniotic (Correct answer)
- Dichorionic-triamniotic
- Monochorionic-triamniotic
- Monochorionic-diamniotic plus one dichorionic sac
Correct answer: Trichorionic-triamniotic
Spontaneous triplets arise from three separately ovulated and fertilized eggs in the vast majority of cases, resulting in trichorionic-triamniotic placentation.
Question 5: What is the standard recommended ultrasound surveillance interval for uncomplicated monochorionic-diamniotic (MCDA) twin pregnancies to screen for TTTS?
- Every 4 weeks throughout the second trimester
- Every 2 weeks starting from 16 weeks of gestation (Correct answer)
- Every week starting at 20 weeks
- Monthly until 28 weeks then biweekly
Correct answer: Every 2 weeks starting from 16 weeks of gestation
MCDA twins require every-2-week ultrasound surveillance beginning at 16 weeks to detect TTTS early, as it most commonly develops between 16 and 26 weeks.
Question 6: Acardiac twinning (twin reversed arterial perfusion — TRAP sequence) occurs exclusively in which type of twin pregnancy?
- Dichorionic-diamniotic twins only
- Monochorionic-diamniotic twins only
- Monochorionic-monoamniotic twins only
- Any monochorionic twin pregnancy regardless of amnionicity (Correct answer)
Correct answer: Any monochorionic twin pregnancy regardless of amnionicity
TRAP sequence requires arterio-arterial anastomoses within a shared monochorionic placenta, so it can occur in any monochorionic pregnancy (MCDA or MCMA).
Question 7: When counting membrane layers to confirm chorionicity, how many layers does a dichorionic-diamniotic inter-twin membrane contain?
- 2 layers (one amnion from each twin)
- 3 layers (two amnions and one chorion)
- 4 layers (two amnions and two chorions) (Correct answer)
- 1 layer (fused amnion)
Correct answer: 4 layers (two amnions and two chorions)
A DCDA inter-twin membrane has four layers: two amnions (one per twin) and two chorions (one per twin), producing a thick membrane visible on ultrasound.
PU Multiple Gestation Ultrasound 2
What defines Quintero Stage II in twin-to-twin transfusion syndrome (TTTS)?