The incidence of monozygotic twinning is three to five per 1000 births, and around two thirds are monochorionic diamniotic twins resulting from division of the early cell mass some three to eight days after fertilisation. Up to one third of monochorionic twin pregnancies are affected by twin-twin transfusion syndrome (TTTS) and this largely accounts for the three to five times higher perinatal mortality and morbidity in monochorionic twins than dichorionic ones.

The original clinical concept of TTTS was that intertwin vascular anastomoses in the monochorionic placenta caused anaemia and growth restriction in the donor twin, whereas polycythaemia and circulatory overload occurred in the recipient. The diagnostic neonatal criteria were an intertwin haemoglobin difference of > 5 g/100 ml and a birthweight discrepancy of > 20%.

Ultrasound diagnostic criteria are now based on presentation in the second trimester with discordant amniotic fluid volume (oligo/polyhydramnios sequence) in monochorionic placentation. Oliguria in the donor twin results in oligohydramnios, which in extreme cases leads to the fetus becoming tightly enwrapped within its membrane and trapped or stuck against the uterine wall (“stuck twin syndrome”). The bladder and stomach are not visible in anhydramnios. The recipient twin develops hypervolaemia, polyuria, and polyhydramnios with a distended bladder. Hydrops and right sided heart failure with tricuspid regurgitation may occur.

Intertwin vascular anastomoses are normal in monochorionic placentas consisting of superficial artery to artery and vein to vein channels within the chorionic plate, and arteriovenous links within shared cotyledons (deep anastomoses). Up to one third of monochorionic twin pregnancies are affected by TTTS, and the probable reason why most escape this condition is that the shunt of blood between twins is bidirectional and balanced.1 Indeed, the occurrence of superficial artery to artery anastomoses detected by colour Doppler was lower in monochorionic twin pregnancies affected by TTTS compared with unaffected …