• Surgical injury to fetal [ Time Frame: at 28 weeks gestation ]
  Using MRI to assess postoperative thermal injury and nerve injury of fetus
• Postoperative complications [ Time Frame: at delivery ]
  Frequency of PPROM, premature delivery, frequency of infection, procedure-to-delivery interval, <28 weeks of intrauterine fetal death/abortion, >28 weeks of intrauterine fetal deat and <32 weeks premature delivery
• Perinatal outcomes [ Time Frame: a week after delivery ]
  Using Apgar Score to assess neonatal asphyxia（Heart rate, respiration, muscle tone, laryngeal reflex and skin color within one minute after birth are taken as the basis, each item is 0~2 points, the full mark is 10 points. 8~10 belong to normal newborns. 4~7 can be classified as mild asphyxiation, 0~3 as severe asphyxiation）
• Growth of the children [ Time Frame: 6 months postpartum ]
  The height of the surviving fetus at 6 months

Complicated pregnancies refer some special complications, including twin to twin transfusion syndrome(TTTS), twin reverse arterial perfusion(TRAP), selective intrauterine fetal growth restriction(sIUGR), twin anemia-polycythemia sequence(TAPS), one fetal death or structural abnormalities, etc. Complicated twins are often associated with high perinatal morbidity and mortality. Severe complicated twins can be treated with fetal-reduction surgery during pregnancy, which can improve the survival rate of retained fetuses and reduce the disability rate. The managements include mechanical umbilical cord ligation or thermal coagulation obliteration, such as bipolar coagulation (BCC), radiofrequency ablation (RFA), and laser coagulation.Nowadays radiofrequency ablation reduction is the most common, which processes a higher survival rate and fewer maternal and fetal complications compared with other ways.Latest experience suggests that microwave ablation(MWA) is also easy to be used and the therapeutic effect is similar with RFA, but there is no convincing evidence. Compared with radiofrequency ablation, MWA has the advantages of simple operation and time, and avoids the problems of tissue carbonization, drying and heat sink effect. For fetation reduction surgery, the main advantage is that the endothermic effect near the blood vessels is smaller, which enables microwave fetation reduction to have a larger ablation range and a higher thermal effect. This study is comparing Microwave Ablation(MWA) and radiofrequency ablation (RFA) methods for selective fetal reduction in the treatment of complicated monochorionic (MC) multifetal gestations.

This is a pilot randomised controlled trial(RCT) of 60 patients who undergo selective fetal reduction. The subjects of this study were all pregnant women and their offspring who underwent reduction surgery in the third hospital of Beijing University, and patients are randomly assigned 1:1 to different intervention groups by parallel design. The outcome evaluators were blind. Collecting their surgery, clinical information and maternal and fetal complications at a week and a month after surgery, 42 days and 6 months after delivery, to verify the safety and efficacy of MWA, and find a better treatment plan for complicated monochorionic pregnancies .The primary outcome was neonatal survival rate.

• Twin; Complicating Pregnancy
• Microwave Ablation

• Procedure: Microwave ablation
  Use of microwave energy to ablate tissue surrounding fetal umbilical cord. All the procedures are performed percutaneously under ultrasound guidance.
• Procedure: Radiofrequency ablation
  Use of radiofrequency energy to ablate tissue surrounding fetal umbilical cord. All the procedures are performed percutaneously under ultrasound guidance.

• Experimental: Microwave ablation group
  Microwave ablation is used for the treatment of Complicated Monochorionic Pregnancies
  Intervention: Procedure: Microwave ablation
• Active Comparator: Radiofrequency ablation group
  Radiofrequency ablation is used for the treatment of Complicated Monochorionic Pregnancies
  Intervention: Procedure: Radiofrequency ablation

Inclusion Criteria:

1. Complicated monochorionic pregnancies women;
2. Having the indication of selective reduction;(TTTS III or IV, TRAP, severe sIUGR, Twin malformation inconsistency or stillbirth，three or more fetuses with monochorionic pregnancies requiring reducing the number of fetuses）
3. The reduction surgery should be done after 15 weeks of gestation;
4. Willingness to participate in the trial and having provided written consent.

Exclusion Criteria:

1. Preoperative examination shows that patient is not appropriate to undergoing reduction surgery, such as acute infection of the organ system, especially the urinary system.
2. Patients need to perform acute reduction surgery due to the progress of the disease and the surgery cannot be scheduled.
3. Other diseases that may affect the experimental results: neuropsychiatric diseases and congenital diseases.