Prevalence:

• 5% of monochorionic twins.
• Results from splitting of the embryonic mass after day 9 of fertilization.

Ultrasound diagnosis:

• Absence of inter-twin membrane. The two umbilical cords insert close to each other with large-caliber anastomoses between the two fetal circulations.
• Color Doppler demonstrates cord entanglement in most cases and this is usually present from the first trimester of pregnancy. Cord entanglement does not contribute to fetal death or brain damage.
• The number of yolk sacs does not accurately predict amnionicity, because a few monoamniotic twins have two yolk sacs and a few diamniotic twins have only one yolk sac.

Associated abnormalities:

• The incidence of chromosomal abnormalities and genetic syndromes is not increased.
• Risk of discordance for structural abnormalities (20%) is higher than in monochorionic diamniotic twins (8%)

Investigations:

• Detailed ultrasound examination.
• Scans at 12 and 16 weeks and then every 2 weeks until delivery. Assessment of growth, brain development, amniotic fluid volume and pulsatility index in the umbilical artery, middle cerebral artery and ductus venosus of both fetuses.
• If there is discordance in fetal size of >15% or any abnormal Dopplers then review every 1 week.

Management:

• Fetal death, usually of both twins, occurs in 70% of cases (50% at <20 weeks’ gestation, 15% at 20-32 weeks and 5% at ≥32 weeks).
• The most likely cause of fetal death, which occurs suddenly and unpredictably, is acute exsanguination across the large anastomoses between the two cords, probably triggered by cord compression.
• Discordance for major abnormality: one option is endoscopic cord occlusion of the affected fetus followed by laser transection of the cord to avoid subsequent death of the healthy twin in later pregnancy.
• TTTS occurs rarely and is suspected by the development of polyhydramnios and the finding of small or absent bladder in the donor and large bladder in the recipient. The proximity of cords precludes the option of endoscopic laser ablation of communicating vessels. Alternative options are amniodrainage, early delivery or endoscopic occlusion and transection of the cord of one of the fetuses.
• There is no need for hospitalization. Delivery should be by elective cesarean section at 32 weeks’ gestation.

Recurrence:

• No increased risk of recurrence.