***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 
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    ***EXAMPLES*** 
    Input: Many strategies have been proposed for the selection of viable embryos for transfer in human assisted reproduction . These have included morphological scoring criteria for 20 , 28 , 44 and 68 h after insemination . The embryo selection is based on morphology , degree of fragmentation and development to the 8-cell . All have shown some correlation with implantation . However , the overall success of these methods is still limited , with over 50 % of all transferred embryos failing to implant . Pronuclear zygote morphology has gained much attention recently due to its positive value in predicting implantation and pregnancy . This prospect i ve study involved 178 conventional IVF patients only . The key aspects of pronuclear scoring and namely the presence of a cytoplasmic halo were related to day 3 of development and morphology in a retrospective study . The Z-score and the presence/absence of a halo had significant effect on the rate of development on day 3 embryo . Low Z-score result in slow development and poor morphology . The absence of a halo also result ed in slow and poor development , low morphology , increased fragmentation Background Embryos selection is crucial to maintain high performance in terms of pregnancy rate , reducing the risk of multiple pregnancy during IVF . Pronuclear and nucleolar characteristics have been proposed as an indicator of embryo development and chromosomal complement in humans , providing information about embryo viability . Methods To correlate the zygote-score with the maternal age and the outcome of pregnancy , we analyzed the pronuclear and nucleolar morphology , the polar body alignment and the zygote configuration in 459 clinical pregnancies obtained by IVF and ICSI in our public clinic in Reggio Emilia , Italy . We derived odds ratios ( OR ) and Corenfield 's 95 % confidence intervals ( CI ) . Continuous variables were compared with Student 's t-test ; P lower than .05 was considered statistically significant . Results We observed a significant increase of " A " pronuclear morphology configuration in 38 - 41 years old patients in comparison to that lower than or equal to 32 years old and a significant decrease of " B " configuration in 38 - 41 years old patients in comparison to that lower than or equal to 32 and in comparison to that of 33 - 37 years old . Related to maternal age we found no significant differences in P1 and in P2 configuration . We found no correlation between zygote-score , embryo cleavage and embryo quality . Conclusions Our results confirm the limited clinical significance of zygote-score suggesting that it can not be associated with maternal age , embryo cleavage and embryo quality . The evaluation of embryo quality based on morphological parameters is probably more predictive than zygote-score OBJECTIVE Both pronuclear morphology and early zygote cleavage have been used in combination with day 3 criteria to predict implantation and pregnancy . However , in routine practice , it is impractical to use both these criteria to select embryos on day 3 . The objective of the present study was to find which of the two criteria is more predictive in terms of implantation and pregnancy . DESIGN R and omized study . SETTING Hospital-based fertility center . PATIENT(S ) A total of 330 IVF/ICSI patient cycles . INTERVENTION(S ) Patients were r and omized to two groups . The embryos of one group were classified into subgroups A , B , and C based on pronuclear morphology ( group 1 ) and the embryos of the second group were classified into subgroups A , B , and C based on early cleavage status ( group 2 ) . MAIN OUTCOME MEASURE(S ) Comparisons were made of implantation and pregnancy rates between groups 1 and 2 , between subgroups within each group , and between the corresponding subgroups of groups 1 and 2 . Progression of zygotes from day 1 to day 3 in group 1 was recorded , and retrospective analysis of pronuclear morphology of zygotes in group 2 was performed . RESULT ( S ) The overall implantation and pregnancy rates tended to be slightly higher for group 1 compared to group 2 patients , but not statistically significant . Further , there was no significant difference between the corresponding subgroups . Implantation and pregnancy rates of subgroup A zygotes from each group were significantly higher ( P<.01 ) than the rates of subgroup C. CONCLUSION ( S ) There was no significant difference between the two groups in terms of implantation and pregnancy . However , pronuclear morphology is a more satisfactory criterion than early cleavage to assist embryo selection on day 3 . This is because zygotes with early cleavage ability can be identified from their pronuclear morphology . Thus , observation for early cleavage on day 1 and assessment of progression of embryos on day 2 in addition to pronuclear morphology scoring is not necessary in the selection of embryos for transfer on day 3 OBJECTIVE To test the hypothesis that pregnancy rates are low if grade Z1 pre-embryos are not available for transfer and to determine if pronuclear morphology is a better predictor of pregnancy than traditional embryo morphology . DESIGN Prospect i ve clinical study . SETTING Academic human reproduction laboratory . PATIENT(S ) One hundred couples undergoing IVF with conventional insemination or ICSI .  INTERVENTION(S ) Embryo quality was assessed using both pre-embryo pronuclear morphology ( zygote scoring or Z-scoring ) at the time of fertilization evaluation and st and ard day 2 and day 3 embryo morphology  ( number of blastomeres and grading based on degree of fragmentation and blastomere size ) . MAIN OUTCOME MEASURE(S ) We tested two decision models , one based on Z-scores and another on morphology , to determine which grading system better predicted pregnancy outcomes in assisted reproductive technique . Zygote score and embryo morphology were measured for all embryos and the transferred embryo pool . Implantation and pregnancy rates result ing from the embryo transfers of all cycles were calculated . RESULT ( S ) The Z-score distribution of 552 embryos was 27 % Z1 , 8 % Z2 , 50 % Z3 , and 15 % Z4 . Z1 and Z3 embryos had significantly ( P approximately .03 ) higher quality over Z2 and Z4 embryos . Using the Z-score decision model with Z1 embryos having highest priority for transfer , pregnancy rates were similar between Z1 and Z3 embryos . Using embryo morphology as a decision model , pregnancy rates were highest in transfers containing one or two " best"- quality embryos . CONCLUSION ( S ) Z1 and Z3 embryos had similar morphology and pregnancy rates . The decision model based on the Z-score model was not better than st and ard embryo morphology in predicting pregnancy outcome OBJECTIVE To verify whether a correlation exists between pronuclear zygote morphology and the chromosomal condition of preimplantation embryos . DESIGN Prospect i ve analysis of pronuclear zygote morphology and preimplantation genetic diagnosis ( PGD ) for aneuploidy of the result ing embryos . SETTING Reproductive medicine unit , day surgery clinic . PATIENT(S ) Seventy-seven patients undergoing 107 PGD cycles because of advanced maternal age ( 77 cycles ) or previous IVF failures ( 30 cycles ) . INTERVENTION(S ) Evaluation of pronuclear zygote morphology and chromosomal condition of the result ing embryos . MAIN OUTCOME MEASURE(S ) Rate of embryo development , proportion of euploid embryos , and distribution of chromosomal abnormalities . The position of pronuclei within the ooplasm , the size and distribution of nucleoli , and the orientation of polar bodies with respect to pronuclei were highly predictive for the presence of complex chromosomal abnormalities in the developing embryos ; zygotes with juxtaposed pronuclei , large-size nucleoli , and polar bodies with small angles subtended by pronuclei and polar bodies were the configurations associated with the highest rates of euploidy . CONCLUSION ( S ) The combination of the patterns related to pronuclear zygote morphology indicated four configurations where the proportion of chromosomally normal embryos was significantly higher compared with the other configurations , suggesting the validity of this scoring system for the selection of embryos generated by PGD patients BACKGROUND The aim was to study zygote evaluation based on pronuclear morphology under the German embryo protection law , according to which only a maximum of three zygotes are allowed to be cultured for embryo transfer . METHODS In this prospect i ve multicentre study , a total of 512 treatment cycles was performed at 10 centres , between November 1999 and October 2000 . Zygotes were classified into seven patterns ( 0A , 0B and 1 - 5 ) . Pattern 0A and 0B zygotes were preferentially used for further culture and transfer . RESULTS Cycles with transfer of at least one embryo derived from pattern 0B , but not pattern 0A , result ed in significantly higher pregnancy ( 37.9 % ) and implantation rates ( 20.5 % ) compared with non-pattern 0B cycles ( 26.4 and 15.7 % ; P < 0.05 and P < 0.01 respectively ) . In younger patients ( aged < or = 35 years ) , significantly more 0B zygotes were available for transfer than in older patients ( 34.2 versus 25.8 % ; P < 0.005 ) . CONCLUSIONS From these data , it is concluded that evaluation of pronuclear morphology is beneficial , especially for countries with legal restrictions regarding embryo selection Background / Aims . Since the assessment s of the morphology of oocytes , zygotes and /or embryos are of crucial importance to select the best c and i date for pregnancy , many morphological evaluation tools have been proposed . Although embryo scoring , particularly cleavage and blastocyst stages , is more convincing due to successful results , zygote scoring still have a bias as different outcomes . In the current study , we design ed a prospect i ve study to test the reliability of  zygote scoring  by focusing on zygote evaluation techniques and its relation with embryo development and embryo selection for transfer . Methods . A total of 1215 mature oocytes from 139 couples were evaluated for the study . Results . There is no correlation between published zygote scoring technique and embryo development . Conclusions . We conclude that the inconsistency of data obtained from zygote scoring might be caused by the static nature of pronuclear stage embryos and thus pronuclear scoring seems to be unreliable evaluation technique for embryo selection BACKGROUND Pronuclear ( PN ) zygote morphology has been proposed as a useful tool for selecting the best embryos for transfer . METHODS PN morphology was recorded in 888 zygotes and classified according to similar/different PN size [ groups A ( n = 816 ) and B ( n = 72 ) ] and to the number , distribution and synchrony of nucleolar precursor bodies ( NPB ) : subgroup I , pronuclei with 3 - 4 polarized NPB ; subgroup II , 5 - 7 synchronic polarized NPB or 7 - 10 NPB distributed r and omly ; and subgroup III , morphologies other than those of groups I or II . Embryo development and chromosomal abnormalities were evaluated for each PN pattern . RESULTS In patients aged < /=37 years , the number of zygotes reaching morula and blastocyst stage was significantly ( P = 0.0003 ) higher in group A than in group B. In group A , the incidence of chromosomal abnormalities was significantly ( P = 0.0247 ) lower than in group B , and significant differences were observed when pattern AI was compared with pattern AII ( P = 0.0280 ) , AIII ( P = 0.0024 ) , BIII ( P = 0.0077 ) and total B ( P = 0.0247 ) . In patients aged > 37 years , statistical differences among groups were not observed . CONCLUSIONS In patients aged < /=37 years , zygotes with similar PN size and with polarized NPB present the best prognosis based on embryo development and the incidence of chromosomal abnormalities , whereas in patients aged > 37 years , this correlation does not exist OBJECTIVE To eluci date the relative predictive value of implantation markers at different stages of preimplantation development . DESIGN Correlation of pronuclear morphology with embryo morphology and implantation rates in retrospective and prospect i ve analysis of  in vitro fertilization/intracytoplasmic sperm injection ( IVF-ICSI )  treatment cycles . SETTING Private infertility center . PATIENT(S ) A total of 441 couples undergoing infertility treatment . INTERVENTION(S ) None . MAIN OUTCOME MEASURE(S ) Size of pronuclei and distance between them , the number and polarization of nucleolus precursor bodies ( NPB ) at the one-cell stage , embryo cleavage and fragmentation rates on days 2 and 3 , and pregnancy and implantation rates . RESULT ( S ) Polarization of the NPB in both pronuclei had a statistically significant correlation with normal membrane breakage during ICSI ( 40 % , compared with 33 % easy , and 31 % difficult membrane breakage ) and also with faster cleavage and lower fragmentation rates of embryos . Sixty-one percent of implanting embryos had polarization of the NPB in both pronuclei compared with 37 % for all embryos . Larger distance between pronuclei and their unequal size had a statistically significant correlation with slower cleavage and inferior embryo quality . Embryo selection based on only pronuclear morphology or on only day-3 embryo morphology yielded implantation rates of 15.1 % and 12.1 % , respectively . Embryo selection based on sequential evaluation of both pronuclear morphology and embryo morphology on day 3 result ed in a 2 
    Output:
    
In conclusion , our findings demonstrate the lack of conclusive data on the clinical efficacy of the  zygote morphology evaluation  in fresh IVF/ICSI cycles , even if biological results showing a good relationship with embryo viability suggest a role in cycles in which the transfer/freezing is performed at day 1