Infertility is an important public health problem, affecting between 8 and 12% of reproductive-age couples, and leading to a high prevalence of childless couples (Bergstrom, 1992; Ombelet and Campo, 2007). The demand for infertility treatment is increasing due to sociocultural factors such as delayed pregnancy, higher incidence of sexually transmitted diseases, obesity rates, and greater acceptance to the available treatments. Recent estimates have shown that more than 1.8 million assisted reproduction treatments (ART) were performed in 2010 and more than 6 million children born were conceived using ART (Dyer et al., 2016).
In vitro fertilization (IVF) techniques have improved performance and currently provide a live embryo transfer rate that is around 25%. However, the success of IVF techniques depends on maximum efficiency in each treatment step, from controlled ovarian stimulation to obtaining adequate numbers of good quality mature eggs, optimal embryo culture conditions, availability of embryos of good quality for transfer and an adequate endometrium for implantation. Changes in clinical and laboratory practices such as the transfer of embryos in the blastocyst stage, improvement of cryopreservation techniques, transfer of thawed embryos after endometrial preparation and single embryo transfer (SET), have been increasingly indicated for patients with good prognosis. At the same time, the use of SET and freezing of all embryos for later transfer of thawed embryos are the major paradigms of ART in recent years (Chambers et al., 2016).
Despite a variation among countries influenced by coverage of health plans, legislation, guides to medical societies, population and culture, in Europe, on average 20% of transfers are single, with up to 69% of cycles in Sweden. In the United States, elective SET is still low, around 10% among women up to 35 years of age (Maheshwari et al., 2011; Practice Committee of the Society for Assisted Reproductive and Practice Committee of the American Society for Reproductive Medicine, 2012). Despite recommendations and increased SET practice, multiple gestation rates in IVF techniques are still high in the overall mean, around 20-25%, as a result of resistance of using SET and maintaining the transfer of multiple embryos with the purpose of obtaining higher gestation rates (Naasan et al., 2012, Kupka et al., 2014, Ishihara et al., 2015). This resistance is due to the idea that transferring more than one embryo may increase the chance of gestation. However, this practice does not take in consideration the increase of maternal and perinatal complications due to multiple pregnancies (Min et al., 2010; Practice Committee of American Society for Reproductive, 2012). In addition, studies have shown that cumulative live birth rates after the transfer of two SET cycles are comparable with a single transfer of two embryos (Pandian et al., 2009; Chambers and Ledger, 2014).
Historically, it is known that among the total number of retrieved oocytes, only a limited amount of them carry the potential to generate full-term gestation. Thus, embryo selection is a crucial step for the success of IVF techniques and the efficiency of this process is related to the decrease in the number of embryos to be transferred, since once it is possible to select embryos with greater implantation potential, the transfer of fewer embryos becomes more feasible and effective. In the routine, the embryonic selection is based on morphological criteria and characteristics of embryo development in order to infer its implantation potential (Ebner et al., 2003).
After controlled ovarian stimulation, approximately 50% of the embryos present chromosomal alterations (Munne et al., 1995). Then, the embryonic genetic evaluation is becoming increasingly used for the selection of euploid embryos for transfer. Blastocyst biopsy associated with chromosomal screening has been shown to be highly predictive of blastocyst developmental potential (Scott et al., 2012), increasing the chances of implantation by selection of euploid blastocysts for transfer in fresh (Scott et al., 2013) and frozen-thawed cycles (Schoolcraft and Katz-Jaffe, 2013).
In addition, preimplantation genetic screening (PGS) is indication for advanced maternal age (Schoolcraft et al., 2009), repeated implantation failure (Blockeel et al., 2008; Rubio et al. 2013), repeat miscarriage (Shahine and Lathi, 2014) and severe male factor (Harper and Sengupta, 2012). More recently, the use of PGS in cases of good prognosis for embryo selection in SET cycles has been proposed for both young and good prognosis patients (Yang et al., 2012), as well as for those in advanced age (Schoolcraft and Katz -Jaffe, 2013), with an important increase in the rate of embryo transfer associated with a reduction in the rates of multiple gestation (Ubaldi et al., 2015).
PGD was initially performed by in fluorescent in situ hybridization (FISH) technology from embryo biopsy on the third day of the development and analysis of one or two blastomeres by evaluating a limited number of chromosomes. The evolution of this technology led to the complete analysis of the 24 chromosomes by the comparative genome hybridization (CGH) platform associated with embryo biopsy in the blastocyst stage and evaluation of a greater number of cells extracted from the trophectoderm. This approach is associated with higher rates of clinical gestation, improved embryo selection in SET cycles and decreasing multiple gestations in patients with good prognosis (Dahdouh et al., 2015). More recently, the next-generation sequencing (NGS) platform has been used and validated for embryonic genetic analysis in IVF, which is used for whole genome amplification (WGA), with the advantage of being a technology with potential to improve the embryonic chromosomal diagnosis in terms of robustness, automation and ability to detect aneuploidies (Fiorentino et al., 2014; Wells et al., 2014).
A recent study developed by the investigators group suggests that patients with good prognosis who perform SET in a fresh cycle and did not become pregnant, do not benefit from receiving a second transfer of two embryos, since the gestation rates are similar to those of patients who received one embryo in the second transfer, with the disadvantage of having a high incidence of multiple pregnancies. These results obtained in a retrospective study already indicate the advantages of performing SET in a sequential manner, promoting satisfactory gestation rates and safety for the mother and baby with regard to multiple gestation for patients with good prognosis (Monteleone et al. , 2016).
Based on these principles, the investigators consider that elective single embryo transfer (SET) associated with preimplantation genetic diagnosis by NGS (NGS + SET) for patients with good prognosis can result in higher success rates of IVF cycles and at the same time decreases multiple pregnancies, as well as maternal-fetal and neonatal risks inherited from this condition. This protocols is suppose to result in high rates of clinical gestation with less occurrence of abortions and greater overall success of the treatment. In order to prove this hypothesis, the investigators propose a study that compares the outcomes of IVF treatments in patients receiving the elective transfer of single embryo genetically evaluated by NGS (NGS + SET) and elective transference of two embryos genetically evaluated by NGS (NGS + DET). As a control, this study will include embryo transfer groups without preimplantation genetic diagnosis by NGS, that is, those that are selected for transfer only from morphological criteria as routine, being associated also the elective transference of one (SET) or two embryos (DET). In case of non-pregnancy in the first cycle of IVF, there will be subsequent transfers of one or two embryos respectively for each group until the remaining embryos are exhausted or until the patient reaches the gestation. The primary outcome will be the cumulative pregnancy rate per treatment cycle. This approach will allow to confirm or not the hypothesis that genetic analysis is effective in improving the embryo selection process and associated with SET will increase the rates of clinical gestation and decrease the rates of multiple gestations and miscarriages.
