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.

Medical charts were reviewed for the details of surgery and the pregnancy outcomes if the patient delivered at the authors'institution. In the event of delivery at distant locations, patients were contacted by telephone to obtain delivery and neonatal information. Patients are randomly assigned 1:1 to different intervention groups by parallel design. Chorionicity was assessed by the referring physicians during the first trimester. All patients underwent comprehensive ultrasound examination to confirm the fetal diagnosis, chorionicity, amnionicity and cervical length upon arrival at Peking University Third Hospital. Monochorionicity was confirmed through the ultrasound findings of a single placenta, a thin intervening membrane, lack of a lambda sign and concordance for fetal gender. Patients were counseled about the risks and benefits of expectant management vs. selective reduction. Written informed consent was obtained. Statistical analysis was performed using the statistical software Statistical Product and Service Solutions(SPSS), for comparison of entry and outcome variables using the chi-square test, Fisher's exact test and the Student's t-test, where appropriate. P < 0.05 was considered as statistically significant.

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.

Requested data for public purpose or research transparency will be provided via the corresponding author. Participant data after deidentification (text, table, figures, and appendices) and protocol will be shared beginning 9 months and ending 36 months following article publication

Participant data after deidentification (text, table, figures, and appendices) and protocol will be shared beginning 9 months and ending 36 months following article publication

Requested data for public purpose or research transparency will be provided via the corresponding author.The e-mail address is weiyuanbysy@163.com.

The statistician will seal the randomisation codes in sequentially numbered opaque envelopes and send them to the research centres. Allocation will be concealed until participants sign the informed consent, at which time the interventionalist will access the allocation code.