[Abstract] In the clinical practice of In Vitro Fertilization and Embryo Transfer (IVF-ET), the failure to establish a pregnancy after at least 3 transfers of embryos is called recurrent implantation failure. Although there is no uniform definition of recurrent implantation failure, the study of its possible etiology has long been a hot topic in reproductive medicine. The etiology of recurrent implantation failure is complex, with abnormalities in gamete and embryo quality and developmental potential, abnormalities in the endometrial microenvironment, and certain systemic factors that can affect both embryo quality and the endometrial microenvironment being possible causes of recurrent embryo implantation failure.
The human reproductive process is considered to be a relatively inefficient one, with women having only a 20-30% chance of pregnancy per cycle, an approximately 50% chance that the embryo will survive beyond 20 weeks of gestation, and a 75% chance of losing the embryo before implantation occurs and a clinical pregnancy is not achieved. Although many scholars agree that the use of exogenous hormones to stimulate the ovaries during In Vitro Fertilization (IVF) produces a large amount of endocrine hormones causing a non-physiological state of the organism that may affect egg maturation and endometrial receptivity, thus leading to a decrease in pregnancy rate; however, due to the efficient culture systems, good embryo selection methods and means, and well established embryo transfer methods used today However, due to the use of efficient culture systems, good embryo selection methods and techniques, and well-established embryo transfer (ET) technologies, the current IVF-ET pregnancy rates have reached or even exceeded those of natural pregnancies [1]. Nevertheless, a significant number of patients in IVF-ET practice still fail to achieve pregnancy after multiple embryo transfers.
Recurrent Implantation Failure (RIF) is defined as the failure of a patient to establish a pregnancy after several consecutive embryo transfers. There is no standard definition of recurrent implantation failure, but some believe that failure to achieve pregnancy after 3 consecutive transfer cycles with 1-2 high-quality embryos is diagnosed as recurrent pregnancy failure; whether a transfer cycle includes a frozen-thawed cycle is also a point of debate; others believe that due to the different age groups of patients in each center and the different treatment protocols used, for example The pregnancy rate of cycles using natural cycles is lower than that of ovulation-promoting cycles, so each center can set its own criteria for recurrent implantation failure based on the pregnancy status of its own center [2].
Factors affecting the successful implantation of embryos can be reflected in both the uterus and the fetus. Some factors may affect the development of the gametes or embryos and lead to implantation failure; some factors may directly or indirectly affect the microenvironment of the endometrium and affect embryo implantation; and some factors may affect both the quality of the embryo and the tolerance of the endometrium and lead to recurrent implantation failure.
I. Factors affecting gamete or embryo development
A decrease in the number and quality of oocytes is an important factor in subsequent embryo development and implantation failure. Poor response to ovarian stimulation, low number of eggs retrieved, high percentage of immature oocytes, low fertilization rate and low embryo utilization all suggest a decrease in egg quality. The low number of eggs retrieved, decreased number of sinus follicles, elevated follicle stimulating hormone and decreased anti-Mullerian hormone suggest decreased ovarian reserve. Decreased ovarian reserve is the main reason for the decrease in egg quality and quantity, which is commonly seen in older women. The increased rate of aneuploidy in older women is indisputable, but there are no convincing reports on how to improve the quality of eggs in older women. PGS is a well-established method to screen for aneuploidy, but its use for screening a small number of embryos in advanced women is controversial [3]. The results of the most recent systematic review and meta-analysis suggest that there are insufficient data to suggest that the addition of genetically recombinant LH, hCG, DHEA and letrozole is beneficial in patients with poor IVF response [4].
It has been suggested that the decrease in embryo implantation rate due to repeated poor embryo quality may also be due to embryo unsuitability for the culture environment, and therefore a 26.4% and 18.9% pregnancy and live birth rate was obtained in 53 patients with repeated IVF failures characterized by multiple low-quality embryos undergoing syngeneic transfer [5].
The results of animal experiments as well as experimental studies with artificial insemination and IVF confirm the association of sperm DNA fragmentation with the success rate of pregnancy. Sperm DNA is bound to ichthyospermins and highly agglutinated to ensure the integrity of its DNA and to protect it from damage during transport in the male reproductive tract and operation in the female reproductive tract. The etiology of sperm DNA damage is multifaceted. Sperm DNA damage can be caused by lack of protection of sperm DNA due to ichthyospermatoid deficiency, oxidative stress produced by varicose veins and leukocytes, smoking, reproductive tract infections, and previous chemotherapy or radiation therapy. Abnormal sperm morphology is also considered an important indicator for evaluating sperm quality, and morphological analysis and analysis of DNA integrity is necessary for sperm in patients with repeated implantation failure, even if the parameters of routine sperm examination are normal. High-resolution selection of sperm morphologically selected for intracytoplasmic injection (IMSI) is considered as one of the methods to improve the pregnancy rate in patients with recurrent implantation failure.
Even when morphologically well selected embryos are transferred into the uterine cavity, there is still a notably low pregnancy failure rate. This may be due to the fact that the embryos themselves do not behave in a culture microenvironment consistent with their intrinsic developmental potential. Solutions to this problem have been reported in the literature with techniques such as gamete intrafallopian tube transfer, serial embryo transfer, embryo co-culture and assisted hatching. The clinical use of these techniques for recurrent implantation failure is currently controversial, but some centers have their own implementation criteria. 2008 American Society for the Practice of Reproductive Medicine committee discussions have concluded that there is no evidence to support the effectiveness of routine or universal use of assisted hatching in all IVF cycles. The new real-time embryo monitoring scoring system should give laboratories more accurate information for selecting high-quality embryos.
It is necessary to check the karyotype of both couples in patients with recurrent implantation failure, and Stern C et al. found a higher incidence of balanced chromosomal translocations in patients with in vitro fertilization-embryo transfer failure than in the control population [6]. Routine chromosomal screening of both partners in patients with recurrent implantation failure has been performed at our center for many years and no significant increase in the incidence rate has been found.
II. Factors affecting endometrial receptivity
In addition to synchronization of the endometrium with the embryo, factors affecting embryo implantation include congenital uterine anomalies acquired uterine anomalies.
Congenital uterine anomalies include Mullerian duct fusion anomalies such as arcuate uterus, double uterus, bicornuate uterus and unicornuate uterus. Longitudinal degenerative defects include complete and incomplete mediastinum of the uterus. Among them, bowed uterus and double uterus may not be associated with infertility, while bicornuate and unicornuate uterus as well as longitudinal degenerative defects may affect pregnancy [7]. 2010 data from our center on 64 patients with uterine anomalies treated with IVF showed that the success rate of IVF was significantly lower in patients with saddle uterus, double uterus and unicornuate uterus than in patients with incomplete longitudinal uterus [8].
Acquired uterine anomalies include endometritis, endometrial polyps, submucosal fibroids, uterine adhesions, endometrial scars, adenomyoma and endometrial hyperplasia, and thin endometrium is also considered to be a factor affecting embryo implantation, but remains controversial. Abnormal endometrial blood flow is also thought to be a cause of repeated implantation failure, but there is a lack of standard techniques for detecting and diagnosing endometrial blood flow.
Reflux of fluid from the fallopian tube into the uterus may wash away unimplanted embryos or cause abnormalities in the local environment of the endometrium that may affect embryo implantation. The literature reports that surgical treatment of tubal effusion can improve the pregnancy rate of patients, but care should be taken to minimize damage to the ovarian vessels and protect ovarian function during surgery. Our clinical experience shows that the pregnancy rate of patients with tubal effusion decreases, and the stoma performed before IVF-ET improves their pregnancy outcome [9].
III. Factors affecting both embryo and uterus
Some systemic factors may affect both egg quality or embryo development, as well as lead to changes in the local immune environment of the endometrium, which may lead to implantation failure.
Pregnancy rates for in vitro fertilization-embryo transfer in obese women with a body mass index greater than 30 are significantly lower than in normal weight women. German researchers analyzed pregnancy data from 650452 cycles over a total of 12 years from 1997-2008 and found that the fresh cycle pregnancy rate was higher in obese male and normal weight female couples than in obese female, normal weight female and normal weight male combination couples, suggesting that the reason for this interesting phenomenon may be due to the higher social status of obese male and normal weight female couples and thus It is considered likely that their increased pregnancy rates could become associated with other lifestyle factors and having a higher social status [10]. It can be hypothesized that successful embryo implantation also involves the effect of socioeconomic stress on the whole organism through the neuro-endocrine-immune axis.
The pregnancy rate decreases in patients with hypothyroidism or hyperthyroidism, while the pregnancy success rate of embryo implantation can be significantly higher after treatment of the controlled condition.
Systemic and uterine local immune cells such as Th cells and their cytokine abnormalities, endometrial NK cell abnormalities [11], antiphospholipid antibodies, and antinuclear antibodies are associated with decreased embryo implantation rates, but the literature is divided and their role in repeated embryo implantation failure is controversial. 2011 American Society for Reproductive Medicine Clinical Practice Committee concluded that intravenous immunoglobulin and leukocyte immunotherapy have not been shown to be effective.
Genetic and acquired thrombogenic disorders, and smoking have also been implicated in recurrent embryo implantation failure. A study comparing plasma fibrinolytic system indicators in 30 patients with RIF, 60 patients with one successful IVF-ET, and 60 women with normal fertility found decreased plasma fibrinolytic activity in patients with RIF [12]. Low molecular weight heparin is effective in the treatment of IVF patients with thrombophilia [13].
In conclusion, recurrent implantation failure, although there is no standard definition, is a relatively common problem in IVF-ET. Each center may have its own definition criteria, and when these criteria are applied the evaluation results do not vary much. The etiology involves the gametes and embryos, the endometrium, and the effect of systemic neuro-endocrine-immune regulation on the endometrial lining or the gamete embryo.