Prevalence:

• 10-15% of monochorionic twins.

Ultrasound diagnosis:

• Estimated weight <5th percentile in the small fetus and ≥25% discordance between the two fetuses.
• The amniotic fluid in the small fetus is reduced and in the other fetus is normal.
• The condition is subdivided into 3 types according to the Doppler finding of the end diastolic flow (EDF) in the umbilical artery of the small fetus:
  • Type I: EDF positive.
  • Type II: EDF absent or reversed
  • Type III: EDF cyclical change from positive to absent and reversed.
• If in the presence of ≥25% estimated weight discordance between the fetuses there is polyhydramnios in the sac of the bigger twin the condition is sFGR with superimposed TTTS.

Associated abnormalities:

• The incidence of chromosomal abnormalities or genetic syndromes is not increased.

Investigations:

• Detailed ultrasound examination.
• Ultrasound scans every 1 week to monitor growth, amniotic fluid volume and pulsatility index in the umbilical artery, middle cerebral artery and ductus venosus of both fetuses.

Management:

• Types I and II with TTTS: endoscopic laser ablation of communicating placental vessels.
• Type I without TTTS: expectant management with close monitoring to define the best time of delivery. If Doppler finding remain normal then elective cesarean section at 34-35 weeks. There is intact survival of both twins in 95% of cases.
• Type II without TTTS: there is a high risk of perinatal death and handicap for both twins.
  • ≥26 weeks: the best management is close monitoring and delivery if the ductus venosus EDF becomes negative or reversed.
  • <26 weeks: the best management is endoscopic laser ablation of communicating placental vessels. Survival of the big baby is 70% and of the small baby depends of ductus venosus EDF: 40% if positive and 10% if negative or reversed. Risk of neonatal cerebral lesions primarily depends on gestational age at delivery and varies from 20% for birth at <26 weeks to 5% for birth at ≥32 weeks. An alternative management is cord occlusion of the small fetus; the survival of the large twin is 90%.
• Type III: the two umbilical cords are adjacent to each other and the behaviour of the pregnancy is similar to that of monoamniotic twins; development of TTTS is rare and sudden unexpected death could occur in 20-30% of cases. Laser surgery may be impossible and the best management is close monitoring and delivery on the basis of ductus venosus EDF in the small fetus. If the ductus venosus EDF is positive elective delivery should be by cesarean section at 32 weeks’ gestation. There is a 20% risk of neonatal cerebral lesions and the risk is greater in the larger than small fetus.

Recurrence:

• No increased risk of recurrence.