***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: Surplus embryos available for cryopreservation in fresh cycles are considered as having good potential for future use . However , the optimal stage of embryo cryopreservation remains unclear . In this study , 1190 patients with surplus embryos on day 3 were divided into two groups : cleavage-stage embryo cryopreservation ( control group ) and blastocyst cryopreservation ( blastocyst group ) . The clinical outcomes of the subsequent warming cycles were evaluated . The proportion of cycles with blastocyst formation was 73.8 % in the blastocyst group . Although in the blastocyst group , the cancellation rate of blastocyst transfer was increased due to lack of blastocysts available for cryopreservation , the blastocyst group achieved significantly higher rates of clinical pregnancy/cycle ( 43.2 % versus 34.9 % ; P=0.003 ) , pregnancy/transfer ( 59.5 % versus 35.4 % ; P<0.001 ) and implantation ( 46.5 % versus 22.2 % ; P<0.001 ) from the first warming cycle compared with the control group . In an embryo-number classified analysis , the clinical pregnancy rate was also higher in the blastocyst group . However , the cumulative pregnancy was similar between the two groups . Blastocyst culture as an embryo selection tool will not improve embryo viability but it will help patients to achieve pregnancy more quickly . Extended culture of surplus embryos to the blastocyst stage for cryopreservation optimizes the clinical outcomes Objective It is well known that fresh blastocyst transfer results in better pregnancy outcomes with a smaller number of transferred embryos compared with cleavage stage embryo transfer . However , in terms of frozen-thawed blastocyst transfer , only a few studies are available . We aim ed to evaluate clinical outcomes of frozen-thawed embryo transfer ( FET ) with blastocysts . Methods Retrospective analysis of FET cycles with blastocysts ( B-FET ) between Jan 2007 and June 2009 was performed . Age-matched FET cycles with cleavage stage embryos ( C-FET ) during the same period were collected as controls . A total of 58 B-FET cycles were compared with 172  C-FET cycles and also compared with those of post-thaw extended culture blastocysts from frozen pronuclear stage embryos ( 22 cycles ) . Results There was no difference in the patient characteristics of each group . The embryos ' survival rates after thawing were comparable ( > 90 % ) and there was no difference in the implantation rate or clinical and ongoing pregnancy rate among the three groups . Conclusion In FET , blastocyst transfers may not present better pregnancy outcomes than cleavage stage embryo transfers . A further large-scale prospect i ve study is needed Conventionally , most in vitro fertilization ( IVF ) embryos are transferred in fresh treatment cycles with freezing reserved for spare ones . Improvement in cryopreservation facilities over time has encouraged the greater use of this technology with the success rate of frozen replacement cycles approaching that associated with fresh embryo transfer . Data from observational studies suggest that obstetric and perinatal outcomes are better in pregnancies result ing from frozen replacement cycles . In the interests of promoting feto-maternal safety is it therefore time to avoid fresh embryo transfers in IVF , freeze all available embryos and replace them in subsequent cycles ? In this article we explore the biological plausibility of this concept , appraise the evidence underpinning it and consider the implication s of adopting such a strategy in routine clinical practice . The outcomes of existing r and omized trials appear to favour a strategy of  frozen embryo transfer  , but larger trials are needed before a major change in clinical practice can be considered High-frequency uterine contractions at the time of non-cavitating embryo transfer influence adversely IVF-embryo transfer outcome . This prompted us to quantify prospect ively the possible decline in uterine contraction frequency occurring during later stages of the luteal phase of ovarian stimulation , up to the time of blastocyst transfers , in 43 IVF-embryo transfer c and i date s.  Contractility was assessed on the day of human chorionic gonadotrophin ( HCG ) administration , 4 days after HCG ( non-cavitating embryo transfer ; HCG + 4 ) , and 7 days after HCG ( blastocyst transfers ; HCG + 7 ) . For this , 2 min sagittal uterine scans were obtained by ultrasound and digitized with a computerized system for the assessment of uterine contraction frequency . Our results indicated that a slight , yet significant , decrease in uterine contraction frequency , observed from the day of HCG ( 4.4 + /- 0.2 contractions/min ) to HCG + 4 ( 3.5 + 0.2 contractions/min ) , was followed by a more pronounced , additional decrease between HCG + 4 and HCG + 7 ( 1.5 + /- 0.2 contractions/min ; P < 0.001 ) . In conclusion , during the luteal phase of ovarian stimulation , uterine contractility decreases progressively , and reaches a nearly quiescent status 7 days after HCG administration , at the time of blastocyst transfers . It is possible that such a uterine relaxation assists blastocyst implantation BACKGROUND Single-embryo transfer has been recommended to reduce the incidence of multiple gestations when in vitro fertilization is performed in women under 36 years of age . We design ed a prospect i ve , r and omized , controlled trial to determine whether there were any differences in the rates of pregnancy and delivery between women undergoing transfer of a single cleavage-stage ( day 3 ) embryo and those undergoing transfer of a single blastocyst-stage ( day 5 ) embryo . METHODS We studied 351 infertile women under 36 years of age who were r and omly assigned to undergo transfer of either a single cleavage-stage embryo ( 176 patients ) or a single blastocyst-stage embryo ( 175 patients ) .  Multifollicular ovarian stimulation was performed with a gonadotropin-releasing hormone antagonist and recombinant follicle-stimulating hormone . RESULTS The study was terminated early after a prespecified interim analysis ( which included 50 percent of the planned number of patients ) found a higher rate of pregnancy among women undergoing transfer of a single blastocyst-stage embryo ( P=0.02 ) . The rate of delivery was also significantly higher in this group than in the group undergoing transfer of a single cleavage-stage embryo ( 32.0 percent vs. 21.6 percent ; relative risk , 1.48 ; 95 percent confidence interval , 1.04 to 2.11 ) . Two multiple births occurred , both of monozygotic twins , both of which were in the group undergoing transfer of a single cleavage-stage embryo . CONCLUSIONS These findings support the transfer of a single blastocyst-stage ( day 5 ) embryo in infertile women under 36 years of age Purpose To determine if blastocyst transfer increases the ongoing and cumulative pregnancy rates , compared with day 3 embryo transfer , in women of all ages when at least 4 zygotes are obtained . Methods Prospect i ve study including patients undergoing a first IVF/ICSI treatment and assigned to cleavage stage ( n = 46 ) or blastocyst ( n = 58 ) embryo transfer .  Supernumerary embryos were vitrified and patients failing to achieve an ongoing pregnancy after fresh embryo transfer would go through cryopreserved cycles . The main outcome measure was the ongoing pregnancy rate after the fresh IVF/ICSI transfer and the cumulative ongoing pregnancy rate . Results were also analyzed according to age ( under 35 and 35 or older ) . Results A majority of patients ( 96.6 % ) had a blastocyst transfer when at least 4 zygotes were obtained . The ongoing pregnancy rate was significantly higher in the day-5 group compared with the day-3 group ( 43.1 % vs. 24 % , p = 0.041 ) . The cumulative ongoing pregnancy rate was higher ( but not significantly ) with blastocyst than with cleavage stage embryos ( 56.8 % vs. 43.4 % , p = 0.174 ) . When analysed by age , patients 35 or older showed significantly higher ongoing pregnancy rate ( 48.4 % vs. 19.3 % , p = 0.016 ) and cumulative ongoing pregnancy rate ( 58 % vs. 25.8 % , p = 0.01 ) in the day-5 group compared to the day-3 group , while no such differences were observed in women under 35 . Conclusions Blastocyst transfer can be suggested whenever there are at least 4 zygotes . While there are no differences in women under 35 , the benefit of this option over cleavage stage transfer could be significant in women 35 or older OBJECTIVE To compare the implantation and pregnancy rates after cleavage stage embryo transfer ( ET ) with transfer of blastocyst-stage ( days 5 - 6 ) embryos . STUDY DESIGN Prospect i ve r and omized trial at an assisted reproduction unit in a university hospital . Women with six or more follicles at the last ultrasound scan before oocyte aspiration were r and omized for transfer of a maximum of two embryos after 2 - 3 days ( n = 80 ) or after 5 - 6 days ( n = 64 ) of culture . Embryo quality , implantation and pregnancy rates were evaluated . Statistical significance was tested with the Chi-square test and Fisher 's exact test . RESULT ( S ) No significant difference was observed in implantation rates ( 21.1 % versus 20.9 % , respectively ) and clinical pregnancy rates ( 36.7 % versus 32.5 % respectively ) after blastocyst and cleavage stage transfers for the two groups . The pregnancy rate among subjects who had at least one good quality embryo transferred was 37.5 % per day 2 - 3 ET and 60 % per day 5 - 6 ET . CONCLUSION ( S ) The overall implantation and pregnancy rates after embryo transfer at cleavage stage and at blastocyst stage transfer were not statistically different . Women who had at least one good quality blastocyst ( n = 25 ) had a high pregnancy rate ( 60 % per ET ) . Blastocyst transfer is a good alternative for couples with many good quality embryos on day 2 after insemination 
    Output:
    
Conclusion In summary , this meta- analysis shows that vitrification at any stage has no detrimental effect on clinical outcome .
Blastocyst transfer will still remain a favorable and promising option in ART .