Prevalence:

• Spontaneous: 5% of monochorionic twins. Usually occurs >26 weeks' gestation.
• After laser ablation of placental vessels: 2-10% of monochorionic twins.

Ultrasound diagnosis:

• TAPS occurs due to a slow transfusion of blood from the donor to the recipient through few very small arteriovenous vascular anastomoses leading to highly discordant hemoglobin levels.
• There is no substantial difference between the twins in either size or amniotic fluid volume.
• The placenta of the anemic fetus looks thick and hyperechogenic, whereas that of the polycythemic fetus looks thin and translucent.
• The fetal middle cerebral artery peak systolic velocity (MCA PSV) in the anemic fetus is increased (>1.5 multiples of median), whereas that in the polycythemic fetus is decreased (<1 multiples of the normal median).
• The anemic fetus may have a dilated heart, tricuspid regurgitation and ascites. The liver of the polycythemic fetus has a starry sky pattern due to diminished echogenicity of the liver parenchyma and an increased brightness of the portal venule walls.

Associated abnormalities:

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

Investigations:

• Detailed ultrasound examination, including echocardiography (to assess cardiac function) and measurement of MCA PSV (to predict degree of anemia and polycytemia).

Management:

• <26 weeks: endoscopic laser ablation of communicating placental vessels. Subsequently, scans every 1 week to monitor fetal growth, brain anatomy and MCA-PSV. Fetal brain MRI at ≥32 weeks’ gestation should be considered for the diagnosis of neuronal migration disorders. If both babies are developing normally vaginal delivery can be carried out at 37 weeks.
• 26-30 weeks: intrauterine blood transfusions to the anemic twin and exchange transfusion with Hartmans’ solution for the polycythemic twin. Doppler assessment every 2-3 days because it may become necessary to undertake interventions every 3-4 days. Delivery by cesarean section at 30-32 weeks.
• >30 weeks: delivery by cesarean section.

Prognosis:

• Neurodevelopmental delay in up to 20% of cases, which mainly depends on gestational age at birth. The risk is higher for the recipient twin.

Recurrence:

• No increased risk of recurrence.