***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: Background Oocyte cryopreservation is an important method used in a number of human fertility circumstances . Here , we compared the survival , in vitro maturation , fertilization , and early embryonic development rates of frozen-thawed human immature oocytes using two different cryopreservation methods . Methods A total of 454 failed-matured oocytes [ germinal vesicle ( GV ) and metaphase I ( MI ) stages ] were collected from 135 patients ( mean age 33.84 + /- 5.0 y ) who underwent intracytoplasmic sperm injection ( ICSI ) cycles between February 2009 and December 2009 and r and omly divided into a slow freezing group [ 1.5 mol/L-1 , 2-propanediol ( PROH ) + 0.2 mol/l sucrose ] and vitrification group [ 20 % PROH + 20 % ethylene glycol ( EG ) + 0.5 mol/l sucrose ] . Results The vitrification protocol yielded a better survival rate than the slow freezing protocol at each maturation stage assessed . Regardless of the maturation stage ( GV + MI ) , the slow freezing protocol had a significantly lower survival rate than the vitrification protocol ( p < 0.001 ) . In addition , a significant difference was found in the survival rates between GV and MI oocytes regardless of the protocol used ( 90.1 vs. 64.7 % , respectively ; p < 0.01 ) . We also found that the maturation rates of GV and MI oocytes from the slow freezing and vitrification groups were 16.7 vs. 24.4 % and 50.8 vs. 55.4 % , respectively . Regardless of the protocol used , the GV oocytes had significantly lower viability than MI oocytes after 36 h of in vitro maturation ( 21.2 vs. 54.0 % , respectively ; p < 0.01 ) . In addition , the GV and MI oocytes from the slow freezing group had a markedly lower maturation rate than those from the vitrification group ( 33.6 vs. 43.1 % , respectively ) , but no statistical difference was found between the two groups ( P > 0.05 ) . For the GV-matured oocytes , no fertilized eggs were obtained in the slow-freezing group , while a 19.0 % ( 4/21 ) fertilization rate was observed in the vitrification group . For the MI-matured oocytes , fertilization rates for the slow freezing and vitrified groups were 36 % and 61.1 % , respectively , but no significant difference was found between the two groups ( PIn the Methods section in the MS , all procedures were compliant with ethical guidelines , i.e. approved by the Ethical Committee of our university and Informed Consent signed by each patient . > 0.05 ) . In the GV vitrification group , no embryo formed ; however , in the MI slow freezing group , 12 oocytes were fertilized , but only two achieved cleavage and were subsequently blocked at the 2-cell stage . In the MI vitrification group , a total of 22 embryos were obtained , five of which developed to the blastocyst stage . Conclusions Vitrification is superior to the slow freezing method in terms of the survival and developmental rates for the cryopreservation of human failed-matured oocytes . In addition , GV oocytes appeared to be more resistant than MI oocytes to the low temperature and cryoprotectant used during cryopreservation BACKGROUND An efficient oocyte cryopreservation method is m and atory to establish a successful egg-banking programme . Although there are increasing reports showing good clinical outcomes after oocyte cryopreservation , there is still a lack of large controlled studies evaluating the effectiveness of oocyte cryo-banking . In this study , we aim ed to compare the outcome of vitrified-banked oocytes with the gold st and ard procedure of employing fresh oocytes . METHODS A r and omized , prospect i ve , triple-blind , single-centre , parallel-group controlled- clinical trial ( NCT00785993 ) , including 600 recipients ( alpha = 0.05 and power of 80 % for sample -size calculation ) selected among 1032 eligible patients from November 2008 to September 2009 , was design ed to compare the outcome of vitrified-banked oocytes with the gold st and ard procedure of employing fresh oocytes . The study was design ed to establish the superiority of the ongoing pregnancy rate ( OPR ) of fresh oocytes over that of vitrified oocytes , by performing a likelihood ratio test in a logistic regression analysis expressed as odds ratio ( OR ) with 95 % confidence interval ( CI ) . A limit of 0.66 for OR of vitrified versus fresh groups was defined to set up a possible conversion from superiority to non-inferiority . R and omization was performed 1:1 based on a computer r and omization list in vitrification ( n = 300 ) or fresh groups ( n = 300 ) . The primary end-point was the OPR per r and omized patient i.e. intention-to-treat population ( ITT ) . Secondary end-points were clinical pregnancy ( CPR ) , implantation ( IR ) and fertilization rates , respectively . Additionally , embryo developmental characteristics were recorded . RESULTS There were no differences in donor ovarian stimulation parameters , demographic baseline characteristics for donors and recipients , ovum donation indications or male factor distribution between groups ( NS ) . The OPR per ITT was 43.7 and 41.7 % in the vitrification and fresh groups , respectively . The OR of OPR was 0.921 in favour of the vitrification group . Nevertheless , the 95 % CI was 0.667 - 1.274 , thus the superiority of fresh group with respect to OPR was not proven ( P = 0.744 ) . Non-inferiority of the vitrified group compared with the fresh group was shown with a margin of 0.667 , which was above the pre-established non-inferiority limit of 0.66 . CPR per cycle ( 50.2 versus 49.8 % ; P = 0.933 ) or per embryo-transfer ( 55.4 versus 55.6 % ; P = 0.974 ) , and IR ( 39.9 versus 40.9 % ; P = 0.745 ) were similar for patients receiving either vitrified or fresh oocytes . The proportion of top- quality embryos obtained either by inseminated oocyte ( 30.8 versus 30.8 % for Day-2 ; and 36.1 versus 37.7 % for Day-3 , respectively ) or by cleaved embryos ( 43.6 versus 43.8 % for Day-2 and 58.4 versus 60.7 % for Day-3 , respectively ) was similar between groups ( NS ) . CONCLUSIONS This controlled-r and omized , clinical trial confirmed the effectiveness of oocyte cryo-storage in an ovum donation programme , failing to demonstrate the superiority of using fresh oocytes with respect to the use of vitrified egg-banked ones in terms of OPR . Instead , the non-inferiority of vitrified oocytes was confirmed . These findings involve highly relevant issues that may open a new range of possibilities in ART STUDY QUESTION How does vitrification affect oocyte viability ? SUMMARY ANSWER Vitrification does not affect oocyte viability in oocyte donation cycles . WHAT IS KNOWN ALREADY Oocyte vitrification is performed routinely and successfully in IVF and oocyte donation programs . STUDY DESIGN , SIZE , DURATION This is a prospect i ve study performed between June 2009 and February 2012 to compare ongoing pregnancy rates and other indices of viability between fresh and vitrified oocytes . A total of 99 donations with more than 16 oocytes ( MII ) in which oocytes were allocated both to a synchronous recipient ( fresh oocytes ) and to an asynchronous recipient ( vitrified oocytes ) were included . PARTICIPANTS / MATERIAL S , SETTING , METHODS The participants were consenting couples ( donors and recipients ) from the oocyte donation program . On the day of retrieval , the oocytes allocated to the synchronous recipient were inseminated and those allocated for banking were denuded of cumulus and vitrified . Vitrified oocytes were microinjected with spermatozoa 2 h after warming . Embryo transfer was performed on Day 2 of development in both groups , and the remaining embryos were cryopreserved on Day 3 . Clinical pregnancy was defined by a positive fetal heartbeat at 6 weeks . MAIN RESULTS AND ROLE OF CHANCE A total of 989 oocytes were warmed and 85.6 % survived . No significant differences were observed between fresh and vitrified oocytes : fertilization rate ( 80.7 versus 78.2 % ) , ongoing embryo rate ( 71.0 versus 68.2 % ) or good- quality embryo rate ( 54.1 versus 49.8 % ) . The mean number of embryos transferred was similar in both groups ( 1.82 ± 0.44 versus 1.90 ± 0.34 ) . The implantation rate ( 33.3 versus 34.0 % ) and the multiple pregnancy rate ( 27.7 versus 20.8 ) were also similar between both groups ( P > 0.05 ) . The live birth rate per cycle was 38.4 % in the recipients of fresh oocytes and 43.4 % in the recipients of vitrified oocytes ( P > 0.05 ) . Eighty five frozen embryo transfers were also evaluated . Comparing embryos from fresh and vitrified oocytes there were no significant differences in the embryo survival rate ( 70.1 versus 65.8 % ) , clinical pregnancy rate ( 40.8 versus 33.3 % ) or implantation rate ( 21.8 versus 26.8 % ) . LIMITATIONS , REASONS FOR CAUTION The oocytes were donated by healthy , young women ( ≤35 years ) and these results can not be extrapolated to other population s. WIDER IMPLICATION S OF THE FINDINGS Outcomes obtained with vitrified oocytes are as good as with fresh oocytes and the use of vitrification can be extended to new applications , e.g. accumulation of oocytes from successive stimulations for preimplantation genetic diagnosis , for patients at risk of ovarian hyperstimulation syndrome or in patients needing to preserve their fertility . STUDY FUNDING /COMPETING INTEREST(S ) This work was done under the auspices of the Càtedra d'Investigació en Obstetrícia i Ginecologia of the Universitat Autònoma de Barcelona Accumulation of oocytes from several ovarian stimulation cycles is currently possible using novel vitrification technologies . This strategy could increase the inseminated cohort , creating a similar situation to normoresponders . This study included 242 low-responder ( LR ) patients ( 594 cycles ) whose mature oocytes were accumulated by vitrification and inseminated simultaneously ( LR-Accu-Vit ) and 482 patients ( 588 cycles ) undergoing IVF/embryo transfer with fresh oocytes in each stimulation cycle ( LR-fresh ) .  Drop-out rate in the LR-fresh group was > 75 % . The embryo-transfer cancellation per patient was significantly lower in the LR-Accu-Vit group ( 9.1 % ) than the LR-fresh group ( 34.0 % ) . Live-birth rate (LBR)/patient was higher in the LR-Accu-Vit group ( 30.2 % ) than the LR-fresh group ( 22.4 % ) . Cumulative LBR/patient was statistically higher in the LR-Accu-Vit group ( 36.4 % ) than the LR-fresh group ( 23.7 % ) and a similar outcome was observed among patients aged ⩾40years ( LR-Accu-Vit 15.8 % versus LR-fresh 7.1 % ) . The LR-Accu-Vit group had more cycles with embryo cryopreservation ( LR-Accu-Vit 28.9 % versus LR-fresh 8.7 % ) . Accumulation of oocytes by vitrification and simultaneous insemination represents a successful alternative for LR patients , yielding comparable success rates to those in normoresponders and avoiding adverse effects of a low response . The accumulation of oocytes from several ovarian stimulation cycles is currently possible with the aid of novel vitrification technologies . This strategy could be useful for low-responder patients , contributing to increase the inseminated cohort and creating a similar situation as in normal responders . According to the results presented herein ( higher live-birth rate per patient treated ) , this strategy represents a successful alternative for low-responder patients , yielding comparable success rates to those in normal responders and avoiding the adverse effects of a low response OBJECTIVE To compare the survival , fertilization , early embryonic development , and meiotic spindle assembly and chromosome alignment in frozen-thawed human oocytes after slow-freezing and vitrification .  DESIGN A r and omized study . SETTING A university-affiliated assisted reproductive center . PATIENT(S ) Donated extra eggs from women undergoing assisted reproduction treatment . INTERVENTION(S ) A total of 605 mature oocytes were divided into a slow-freezing group and a vitrification group for cryopreservation . MAIN OUTCOME ME 
    Output:
    
Oocyte vitrification compared to slow freezing probably increases clinical pregnancy rates in women undergoing assisted reproduction .