The application of controlled ovarian hyperstimulation in assisted reproductive medicine makes it possible to obtain sufficient number of oocytes during one menstrual cycle. The number of embryos formed after fertilization often exceeds the number required for one embryo transfer, so the remaining embryos need to be stored frozen, which contributed to the occurrence and development of vitrification and warming technology. Frozen embryo transfer cycle enables patients to obtain more opportunities of embryo transfer without repeated oocyte retrieval, thereby avoiding the economic and mental burdens of the patients and maximizing the use of embryos. More importantly, frozen embryos transfer could effectively avoid the occurrence of ovarian hyperstimulation syndrome (OHSS). In a fresh embryo transfer cycle, the process of controlled ovarian hyperstimulation (COH) affects the hormone secretion. High levels of estrogen may lead to early endometrial maturity and affect endometrial receptivity, thus resulting in adverse perinatal and neonatal outcomes[6–7]. Although frozen-thawed embryo transfer cycle can avoid hyper stimulation of gonadotropins and provide a better microenvironment for embryo implantation in the uterine cavity[8], the process of embryo freezing may cause blastomere damage, and the impact on maternal and neonatal outcomes still remains controversial.
Osamu Ishihara et al. conducted a retrospective study of 3047 fresh single blastocyst transfer cycles and 11329 frozen-thawed single blastocyst transfer cycles, and found that compared with fresh single blastocyst transfer, frozen-thawed single blastocyst transfer could result in similar pregnancy outcomes without increasing the miscarriage rate[9]. However, Daimin Wei et al. have found that the embryo implantation rate, biochemical pregnancy rate and live birth rate of frozen-thawed single blastocyst transfer were significantly higher (P༜0.05) than that of fresh single blastocyst transfer through a large sample, multi-center, randomized controlled clinical study[10]. In this study, the enrolled patients were all no more than 35 years old and treated with IVF for the first time, which could effectively avoid the disadvantages of the advanced age, decreased ovarian reserve and repeated implantation failure. The results showed that both fresh single blastocyst transfer and frozen-thawed single blastocyst transfer could achieve satisfactory pregnancy outcomes. Also, there were no significant differences in adverse pregnancy outcomes such as biochemical pregnancy and late miscarriage rate between the two groups. However, it should be noted that the early miscarriage rate of frozen-thawed single blastocyst transfer was significantly higher than that of fresh single blastocyst transfer(P < 0.05). This conclusion conflicts with the researches mentioned above, the reason may be that early miscarriage is generally related to the quality of the embryo. Vitrification and warming procedures of the embryo may affect the embryonic activity to varying degrees. Studies have found that the incidence of chromosome abnormalities in frozen-thawed embryos is significantly higher than that in the fresh embryos[11]. Although embryo frozen-thawed procedures are developing towards higher efficiency and safety, the potential risks are still not negligible.
The incidence of ectopic pregnancy in IVF patients has increased nearly three times compared with normally conceived females, which ranges between 2.1%~8.6% [12–13].The main reason is that tubal infertility accounts for a large part of the infertility. Also, studies have shown that the incidence of ectopic pregnancy in women with tubal infertility resorting to IVF-ET could be up to 11%[14]. Moreover, the number of embryo transferred in the early days is often ≥ 2. While improving the pregnancy outcomes, it also increases the incidence of heterotopic pregnancy. Compared with embryos in cleavage stage, blastocysts stay in the uterine cavity for a shorter period of time, thereby reducing the probability of migration to the fallopian tube and the occurrence of ectopic pregnancy. Some researchers have concluded that single blastocyst transfer in the frozen-thawed cycle could significantly reduce the risk of ectopic pregnancy[9], which indicated that the frozen-thawed cycle might be closer to a natural state[15–16]. The increased ectopic pregnancy rate in fresh embryo transfer cycle might be due to the hyperphysiological hormonal environment caused by ovarian stimulation[17]. In this study, three cases of ectopic pregnancy occurred in the fresh single blastocyst transfer group, but no case occurred in the frozen-thawed group. The incidence of ectopic pregnancy in both groups was lower than that of natural pregnancy, with no statistical difference ( P༞0.05), which suggested that single blastocyst transfer could reduce the incidence of ectopic pregnancy. For people with high risk factors such as tubal infertility and previous history of ectopic pregnancy, single blastocyst transfer is recommended, among which the frozen-thawed cycle is more preferred.
OHSS is an iatrogenic complication caused by ovarian hyperstimulation with exogenous gonadotropins, which can be life-threatening in severe cases. The incidence in IVF-ET is about 1–10%[18]. In order to prevent OHSS, some patients choose the freeze-all approach and wait for the next frozen-thawed cycle to reduce the production of endogenous hCG, thereby reducing the generation of vasoactive substances.. In this study, the median number of oocyte obtained in the fresh single blastocyst transfer group was 13 and there were 3 cases of moderate to severe OHSS, while the median number of oocytes obtained in the frozen-thawed group was 17 and no case of moderate to severe OHSS occurred. The number of oocyte retrieved in the two groups was statistically different, which further reminds us that for patients with ovarian hyperresponse or with high risk factors of OHSS, frozen-thawed single blastocyst transfer may be a good choice as it can not only avoid the occurrence of OHSS but also obtain the same satisfactory clinical outcome as in the fresh cycle.
It has been reported that the birth defect rate is higher in the frozen-thawed cycle[19], but some scholars[20] found that the frozen-thawed cycle actually did not significantly increase the birth defect rate in singletons. In this study, the birth defects in the fresh single blastocyst transfer group included cardiac malformations (2 cases), hand malformation (1 case), pulmonary sequestration (1 case), ear malformation (1 case), biliary atresia (1 case) and favism (1 case), while in the frozen-thawed group, it was consisted of hypospadias (1 case) and congenital torticollis (1 case). There was one case of newborn death in both groups. No statistical differences were found between the two groups in terms of birth defect rate and neonatal death rate(P > 0.05), but still lacks of large sample randomized controlled studies to confirm.
With regard to the obstetric complications, the incidence of gestational hypertension in frozen-thawed embryo transfer cycle was significantly increased compared with fresh embryo transfer cycle according to a meta-analysis[21]. This study has not yet found that frozen-thawed single blastocyst transfer would increase the risk of obstetric complications. Since whether frozen-thawed cycle would increase the risk of obstetric complications is still controversial, it could not be concluded at present. The meta-analysis also indicated that the incidence of macrosomia and large for gestational age in frozen-thawed embryo transfer cycle was higher than that of fresh embryo transfer cycle, while the incidence of premature, low birth weight infants, and small for gestational age were lower in the frozen-thawed cycle. There was a difference in neonatal birth weight between the two groups. Some researchers[22] believed that this might be related to the epigenetic changes during frozen-thawed embryo transfer and maternal hyperestrogenemia during fresh embryo transfer. However, in this study, the neonatal birth weight, neonatal sex ratio, incidence of macrosomia, small for gestational age and preterm birth were comparable between the two groups (P༞0.05). The reason might be that in the process of ovarian stimulation, for patients with ovarian hyperresponse or high risk factors of OHSS, it was usually recommended that patients give up fresh embryo transfer and consider freeze-all approach to reduce the adverse pregnancy outcomes caused by maternal hyperestrogenemia. In addition, single blastocyst transfer also greatly reduce the incidence of multiple pregnancy, thereby further reducing the occurrence of adverse pregnancy outcomes such as preterm birth and small for gestational age. These results provide new ideas for explaining the differences between the frozen-thawed and fresh single blastocyst transfer, which still need further research.