Interpretation
Female age and quality of transferred embryos were usually accepted as the two most important causes of bias in assay comparisons. In the study, IR, CPR, MPR and LBR were decreased significantly and AR increased significantly with age. The differences of IR, CPR, AR, LBR and EPR between groups S and L were significant in < 35 age group, while the differences of these rates were almost similar in the groups 35-39 and > 39. These results indicated female age was a critical impact factor for clinical outcomes, and different female age distribution of patients in different studies may be connected with different study results. Previous studies demonstrated that D3 embryos with ≤ 6 blastomeres were associated with low IR, CPR, and LBR and high AR compared with those with > 6 blastomeres.10, 12, 16 So, the quality of transferred embryos may be another factor contributing to the discrepancy in different studies. One limitation of this study was that patients’ characteristic of the number of oocytes from the OPU was not mentioned. Because, the limited number of oocytes available after ovarian stimulation was usually associated with advanced maternal age and/or premature ovarian insufficiency, and dramatically decrease of the number and implantation potential of oocytes were together with a dramatic pick up of aneuploidy rates over 70%.17 In addition, women with advanced maternal age had higher risk for miscarriage (over 40%) as well as drastically lower rate of clinical pregnancy.17Therefore, the limited number of oocytes available after ovarian stimulation may be an important risk factor affecting the LBR.
With the development of in vitro culture technology, elective single blastocyst transfer has become a tendency, and this practice has led to a remarkable increase in LBR and decrease in MPR.18, 19 However, the failure to form blastocysts in patients with few day 3 cleavage stage embryos would lead to an increase in cycle cancelation. Therefore, a cleavage transfer policy is still adopted in most centers, as this practice can be used for a wider range of infertility patients. Two or three cleavage stage embryos are transferred in traditional cleavage-stage transfer policy which increases the CPR with the occurrence of high MPR at the same time. Previous study found that morula embryos, developed from post-thawed cleavage embryos, resulted in a higher IR, CPR and LBR than non-morula embryos in FET cycles,20 and extended culture of D3 embryos with ≤ 6 blastomeres to blastocysts, particularly 6-cell embryos, resulted in a similar clinical pregnancy rate as that of blastocysts derived from D3 embryos with ≥ 7 blastomeres.12 The CPR, IR, and LBR were similar in the elective single morula embryo transfer group and elective single blastocyst embryo transfer group.21, 22 A trend toward higher clinical and ongoing pregnancy rates was noted after day 4 embryo transfers when compared with day 3 embryos in ET cycles.23 The development of the fair embryos transferred after 20-22 h in the culture had a highly positive impact on the pregnancy rate.24 Our study also found that transferred embryos with more blastomere growth had a higher IR, CPR, MPR and LBR. The higher MPR in the > 2 group (47.5%) caught our attention and suggested that the embryos with more blastomere growth number were associated with better embryo viability. Even if two embryos with less blastomere growth number (≤ 2 group) were transferred, the MPR remained high (29.0%). Therefore, transferring a well-developed embryo after long post-thawed culture period may be a viable embryo transfer strategy to decrease MPR and cycle cancellation rate while maintaining CPR and LBR. The reasons of higher IR in the L group may be that embryos with hardened zona pellucida have a relatively long time to hatch out and implant in the synchronous endometrium. Moreover, the development of embryos in vitro lags behind that in vivo, so earlier thawing is beneficial to synchronize the development of embryos and endometrium.
Neonatal characteristics of gestational age, preterm birth rates, cesarean section rate, birthweight, low birthweight rate, sex ratio and congenital malformation rate were similar in the long culture period group compared with these in the short culture period group. These results indicated that long post-thawed culture period might be harmless to the clinical outcomes of FET compared with short post-thawed culture period. Multiple pregnancies are considered the most significant adverse event associated with assisted reproductive technologies and is linked to an increased risk of maternal and neonatal morbidity. The study also showed the twin group had a lower average gestational age and birthweight as well as a higher cesarean section rate, preterm labor rate and low birthweight rate than the singleton group. Therefore, elective single well-developed embryo transfer after long post-thawed culture period or elective single blastocyst transfer after blastocyst culture are recommended to decrease the risk of maternal and neonatal morbidity caused by multiple pregnancy.