Multi-center open-label randomized controlled trial to assess if early intervention (12.0-14.0 weeks) (study group) improves the outcome of TRAP sequence as compared to late intervention (16.0-19.0 weeks) (control group). The investigators will randomly assign women diagnosed with TRAP sequence diagnosed between 12.0 and 13.6 weeks to an early or late intervention group (1:1), using a web-based application and a computer-generated list with random permuted blocks of sizes 2 or 4 (www.sealedenvelope.com), stratified by gestational age (GA) at inclusion (11.6 -12.6 weeks versus 13.0-13.6 weeks). Analysis will be by intention to treat.

The investigators propose to conduct a multi-center open-label randomized controlled trial to assess if early intervention (12.0-14.0 weeks) (study group) improves the outcome of TRAP sequence as compared to late intervention (16.0-19.0 weeks) (control group). The investigators will randomly assign women diagnosed with TRAP sequence diagnosed between 11.6 and 13.6 weeks (1:1) to an early or late intervention group, using a web-based application (www.sealedenvelope.com) with a computer-generated list with random permuted blocks of sizes 2 or 4, stratified by gestational age at inclusion (11.6 -12.6 weeks versus 13.0-13.6 weeks). Analysis will be by intention-to-treat. Outcome will be adjudicated blinded to group allocation.

All interventions will be done under local anaesthesia and/or conscious sedation in sterile conditions by an experienced operator. They must be performed within 1 week after randomisation and at the latest at 14.0 weeks in the early group and 19.0 weeks in the late group. In the early group, only intrafetal coagulation will be used. Intrafetal ablation will be performed under ultrasound guidance using an 18-gauge (1.27 mm) to 20-gauge (0.91 mm) needle with a free-hand technique. The needle is introduced into the pelvis/abdomen of the TRAP mass close to the intra-abdominal portion of the feeding vessel, while avoiding puncture of the placenta and pump twin sac. The procedure is considered successful when there is complete cessation of reverse flow into the TRAP mass on intraoperative color-flow mapping.

In the late intervention/control group either intrafetal coagulation or fetoscopic laser coagulation will be performed of the cord and/or anastomosing vessels, unless the flow has stopped spontaneously or demise of the pump twin has occurred in the meantime. Intrafetal coagulation is done as described above by using a 17-gauge (1.47 mm) to 20-gauge needle. Alternatively, fetoscopic laser coagulation of the cord or anastomosing vessels can be performed through a 17-gauge to 7 French trocar with 1-1,3 mm fetoscope and 400 μm laser fiber. The rationale not to standardize the technique in the late intervention group is that several techniques have been reported for treatment after 16 weeks without any significant differences in outcome. Also, it is usual for the surgeon to adapt the technique to the requirements of each individual case, e.g. for a posterior placenta, the surgeon may prefer fetoscopic rather than intrafetal coagulation. Not restricting the technique to only 1 option will therefore more truly represent current practice and increase the generalizability of the trial's findings.

Patients will be discharged the same day or 1 day after the procedure. Management and follow-up will be similar for the study and the control or current practice group. A follow-up scan is usually performed 1 week after the intervention to check for fetal well-being and exclude anemia. A detailed ultrasound scan will be arranged in a fetal medicine center at 20 and 30 weeks to assess the heart and brain anatomy. Some centers may offer an MRI scan at around 30 weeks as part of the protocol for monochorionic twin pregnancies that underwent an intrauterine intervention. Antenatal, peripartum and postnatal care of the mother will be similar to that of a singleton pregnancy and at the discretion of the referring physician. Intrauterine intervention for TRAP sequence is not an indication for cesarean or elective preterm birth.

Inclusion Criteria:

• TRAP sequence in a monochorionic diamniotic twin pregnancy diagnosed between 11.6 and 13.6 weeks, as determined by the crown-rump length of the pump twin in spontaneous conceptions and by the date of insemination or embryonic age at replacement in pregnancies resulting from subfertility treatment
• Women aged 18 years or more, who are able to consent
• Anatomically normal pump twin
• Provide written informed consent to participate in this randomized controlled trial, forms being approved by the Ethical Committees

Exclusion Criteria:

• Contraindication for an intervention due to a severe maternal medical condition or threatening miscarriage
• Inaccessibility of the acardiac twin due to a retroverted uterus, severe maternal obesity, uterine fibroids, bowel or placental superposition
• A major anomaly in the pump twin, requiring surgery or leading to infant death or severe handicap
• Spontaneous arrest of the reverse flow and/or pump twin demise at diagnosis