This study intends to carry out a prospective, randomized, double blind and controlled study to compare the influence of Atosiban and placebo on uterine contraction frequency, endometrial blood flow perfusion, oxytocin and serum concentration of PGF2α, embryo implantation rate and clinical pregnancy rate on the RIF population after fresh embryo transfer, so as to further clarify the curative effect of Atosiban in the treatment of RIF and provide evidence-based basis for Atosiban for application in RIF population.

Background and Rationale With the rapid development of the technology of reproductive medicine, in vitro fertilization embryo transfer (IVF-ET) and its related derivatives have become the most important means for the treatment of infertility. IVF-ET pregnancy rate increased from the initial 10% to current more than 50%, but the repeated implantation failure (RIF) has occurred occasionally, which has become a difficult that perplex IVF-ET clinical practices. There is no uniform definition of RIF at present. Coughlan et al called that the women below 40 years old who experienced at least 3 fresh or frozen periods and failed to transplant 4 and more than 4 high-quality embryos as RIF in 2014 [1]. At present, most of the patients have started the reason screening and inspection spontaneously if the transplantation fails after of transplanting 2 high-quality embryos. But the embryo implantation is a complex process, the etiology of RIF can be roughly summed as the embryo factors, uterine factors, genetic factors, immunological factors and so on, the symptomatic treatment according to different causes can improve the success rate of re-transplantation system [2]. However, in most cases, the etiology of RIF still cannot be explained [3].

In recent years, the influence of uterine contraction on embryo implantation has attracted more and more attention. Studies have shown that no matter it's natural menstrual cycle or fertility cycle, moderate Uterine Contraction (UC) is conducive to embryo implantation, but excessive or strong UC will have a negative impact on embryo implantation, and even the embryos to be implanted to the fallopian tube, cervical or vaginal, and even be discharged of the uterus [4-6].

The estrogen levels of excessively physiological state during the process of ovulation induction of IVF can induce the production ofoxytocin in the endometrial cells, and indirectly lead to the synthesis and release of prostaglandin PGF2α, resulting in increased uterine contraction frequency [7-9]. In 1998, Fanchin et al. found that the frequency of uterine contraction of about 30% of the embryos transplantation patients was higher than 5 times /min, which was significantly correlated to the success rate of low pregnancy [5]. The follow-up studies confirmed by ultrasound showed that the frequency of endometrial contraction induced by ovulation induction cycle of IVF[10] is 5-6 times of the natural cycle. Therefore, in addition to the soft operation of the transplant process, the reduction of uterine excessive contraction by drugs may be an effective measure to improve the success rate of IVF pregnancy.

Atosiban is the antagonist of mixed receptor of pitressin VIA and oxytocin, it competes the oxytocin receptor located on the uterine muscle cell membranes, foetal membrane and deciduas with oxytocin to inhibit contraction of the uterus; at the same time, it inhibits the generation of oxytocin induced uterine endometrial prostate element PGF2 α to increase the endometrial blood flow perfusion. Pierzynski et al first applied Atosiban to the field of reproduction in 2007 for the first time, which made the uterine contraction frequency of a RIF patient who failed to transplant for seven times before the oocyte donation embryo transplant decreased significantly, and successful became pregnant [11]. This report immediately caused the reproductive scientists to apply Atosiban to the clinical study of in vitro assisted reproduction. For the general population, there are still disputes on whether use Atosiban during the embryo transplant [12-14]. The prospective and randomized study of He Ye et al showed that Atosiban can significantly reduce the oxytocin of patients with endometriosis and serum concentrations of PGF2 α, reduce the frequency of uterine contraction and improve the implantation rate of quality blastocyst after freeze-thaw treatment and clinical pregnancy rate [15]. In the RIF population, the retrospective study and prospective cohort studies have shown that using low doses of Atosiban during IVF-ET can increase the implantation rate of fresh and thawed quality embryos and clinical pregnancy rate [16-18], which may improve pregnancy outcomes in patients with RIF. But these studies were not randomized placebo-controlled studies, and their conclusions are still to be verified.

Atosiban is currently a safe and effective tocolytic drug for uncomplicated preterm patients, the patients had accelerated heartbeat, nausea, vomiting, headache, dizziness, flushing, anxiety, tremor and other side effects occasionally. Randomized and double blind controlled trial showed that the side effects with application of Atosiban and placebo in the maternal and child was similar, the difference was not statistically significant [15]. Preclinical studies haven't found that Atosiban has any toxicity to human sperm motility or embryo development in rabbits [20]. It hasn't found the literatures that report the fetal congenital malformations associated with the application of Atosiban. Therefore, the therapeutic dose of Atosiban should be able to be safely used in embryo transfer.

In summary, this study intends to carry out a prospective, randomized, double blind and controlled study to compare the influence of Atosiban and placebo on uterine contraction frequency, endometrial blood flow perfusion, oxytocin and serum concentration of PGF2α, embryo implantation rate and clinical pregnancy rate on the RIF population after fresh embryo transfer, so as to further clarify the curative effect of Atosiban in the treatment of RIF and provide evidence-based basis for Atosiban for application in RIF population.

Inclusion Criteria:

1. 3 or more than 3 embryo implantation failure histories previously (or the number of high-quality embryos transplanted is >4);
2. Age <40 years；
3. Ultrasound or HSG showed normal uterine cavity, endometrium on transplantation day is ≥ 8mm;
4. There's at least one good quality embryo on transplantation day.

Exclusion Criteria:

1. Patients using donor sperm or donor eggs;
2. Patients have obvious uterine cavity abnormalities;
3. There was a clear hydrosalpinx;
4. Natural cycles or IVM patients;
5. Patients who cancel the due to the transfer due to various reasons, such as the failure of fertilization, or ovarian transitional stimulus syndrome (OHSS);
6. Abnormal karyotype;
7. Blastocyst transplantation patients.